Gygax may have only meant the "spirit of adventure" there, but specific rulesets are what provides that unique defining style of gameplay through characters and their abilities, not direct player skills. That creates distinct different roles and options, choices and consequences. When the player skill grows in influence it unavoidably lesses the characters stats influence. Which lessens the spirit.
There are different ways of rewarding player skill. Twitchy ARPG combat is the wrong answer for D&D, of course, but promoting clever/skillful decision-making doesn't lessen the original spirit, it does the opposite. The way 3E de-emphasized player skill is one of the reasons it's so different to the originals, and one of the core principles of the OSR is "player skill over character skill".
Gygax had much to say about that and other factors that made 3E so opposed to the original style and spirit:
Gary Gygax said:
The major appeal of the FRPG is the fantastic, the assumption of a character role in a world filled with strange creatures, and by dint of effort, building through deeds of action and intellect that game persona from a lowly adventurer to a renowned figure with power and prestiege in his milieu. There is little satisfaction in such accomplishment if it isn't earned.
The basis for the D&D game, including 3E and 3.5E is not the superheroic, but the heroic. IMO, the new system hands players on a proverbial silver platter what once had to be earned, and so there is an escalation in character powers and those of "monsters" as those who play the new game seek to find the satisfaction they will never gain from it, because there is no earning of rank through long play that gives actual experience and understanding, grants the ability of clever play, not mere use of gifted powers.
Gary Gygax said:
In OAD&D there was plenty of play aimed at power, just as there is in 3E. Of course those that I knew as "good" players aimed first and foremost at having fun playing the game, regardless of rise in rank and all the rest that goes with power gaming. The challenge of each session was enjoyed more from a group perspective, likely. As the team prospered, so too the enjoyment, cameraderie, and resulting stories. Many a group downplayed combat, developed campaigns in which roleplay was the key. Politics and economics? Sure. While OAD&D certainly focused on combat mechanics and rules, it did not hinder other sorts of play. The XP system in 3E does that with a vengence.
Gary Gygax said:
IMO there has been a vast shift in game focus in 3E. The archetype has gone by the board, comic book-like feats are a feature, the whole purpose of play is set on killing things, and power gaming is encouraged.
Gary Gygax said:
There is no relationship between 3E and original D&D, or OAD&D for that matter. Different games, style, and spirit.
Gary Gygax said:
Also, the way that skills and feats are presented tends to both encourage multi-classing while blurring the archetypes. Yes, one can still play 3E with archetypes, but the system does not encourage it, nor is that in the spirit of the system--gaining power.
Gary Gygax said:
My opinion is this: I think 3E is made for power gamers and relies on seek & destroy for its appeal, so having verisimilitude in any setting in which the system is employed is not of much importance. The changes made in 3E simply up the power of humans, making them more like supermen than the older systems allowed.
Gary Gygax said:
New D&D is a very different game from O/AD&D, and there's no possible debate regarding that.
As the original author of the two systems from which new D&D springs, I must concur. the new version is not in the spirit of those games. That new D&D is played and enjoyed my many gamers means it is unquestionably a good game in its own right, but it is "D&D" in name only when compared to its progenitors.
Gary Gygax said:
Any able DM can craft adventures that weed out unwise and inept players who think to bulldoze their way through problems by use of undeserved power. That’s possible only in computer games where saved games and cheat codes serve to reward such play.
Gygax thought Troika/Cain understood the adventure's spirit, but that didn't make the game "faithful" (Gygax never said that, nor did I). Had Gygax played ToEE, he certainly would've noticed how much the adventure is brought down by the ruleset (not to mention Troika's take on the adventure, which is a different issue). Still, it's not a clear cut issue. Different rulesets can have a similar spirit, and the same ruleset can be used to achieve a different one. For instance, AD&D, LA, and C&C have very different rulesets, but Gygax said all three have the same spirit. On the other end, there is LotFP, which has similar rules to OD&D but a completely different style. Arcana Unearthed/Evolved uses 3E as basis in a very different way, and that one is Gygax-approved.
So again, where does one draw the line? Most Codex users have a clear preference for 3.5E, but one could reasonably argue that a 3.5E game cannot be "faithful to D&D", no matter how perfect the adaptation is. Many draw the line at turn-based combat, but AD&D is phase-based, so how could TB be a requirement? I do think it's important to discuss and preserve D&D's original spirit, but there are very few mechanics that are sine qua nons (attack rolls are an obvious one, Swen is undeniably wrong). OD&D, AD&D, 3E, 4E, and 5E are all so different that everyone's requirements will be biased towards a specific edition/game, so the attempt to make this a binary thing is pointless. It's better to take each for what they're worth and judge their respective adaptations accordingly.
BG3 is different because the title alone ensures it'll be compared to the originals, which had their own style and spirit, while also adapting a different ruleset. It'll be hard for Larian to walk that line, but they chose that title to take advantage of the legacy carved by the originals, so it's only fair that the bar is set higher.