it depends on what you define as "exploration". what is the difference between mapping out a dungeon and mapping out an overworld? what if the game features both, which one is more indicative of the "exploration" aspect?
Oh, most definetely. Dungeon exploration is clearly a valid form of exploration. In fact, I would say that the state of the art in RPG dungeon exploration has always been more advanced than the state of art in overworld exploration. CRPG Dungeon crawling was already excellent in Proving Grounds of the Mad Overlord, and only got better with the refinements on the formula made first by Sir-Tech and then by Starfish. Unfortunately things did not work out as well for overworld exploration. Things started out promising enough with Daggerfall, Darklands, and Might and Magic. Morrowind further pushed the boundaries of how it could be done, but then... Oblivion came around and well, the rest is history. If you look at modern games that are tentatively about exploration or "open-world", what you have is games like Witcher 3 that in fact have more in common with linear Bioware games, but which happen to have a lot of empty spaces thrown in for some reason. Piranha Bytes games and Dragon's Dogma are not really true sandboxes either, not in the same way either DF or MW were. As for the old pioneers, Darklands never had a successor (well, I have heard SitS is kind of like it, but can't vouch for that as I haven't tried it), Might and Magic had an abortive revival which wasn't even faithful to the exploration-oriented nature of the old games (MMX), and while Bethesda has recovered from the darkest hour that was Oblivion/Fallout 3, it's newer games don't really fulfill the potential shown by DF and MW.
I should have specified that I was talking about overworld exploration instead of dungeon exploration when I discussed Bethesda above. They are clearly distinct. Though Daggerfall had plenty of dungeons, but they were procedurally generated and thus had more in common with roguelikes than Wiz-type crawlers. Non-dungeon exploration in DF often also took place in cities rather than in an overworld proper. Still, I would insist that dungeon exploration and non-dungeon exploration are distinct enough categories that they warrant to be examined separately, even if they do share some things in common, such as for example, the fact that they both benefit heavily from both traps and survival/resource management mechanics to be challenging. Dungeon crawlers have traditionally been better at this than overworld games, as they routinely implement hardcore features like saving in town only (some forgo it such as Dungeon Travelers 2 and Dark Spire, though), no fast travel(though there are sometimes teleport spells, but this is valid from the point of view of RPG options) and resting in town only. Heck, Proving Grounds of the Mad Overlord even has ironman by default - except for the Playstation version IIRC.
I wouldn't say overworld/urban exploration is simply worse all-around to dungeon crawling though. It does have some advantages such as the freedom that Morrowind gives you to do all sorts of crazy things that I mentioned a few posts back in my analysis of that game. Also, there is no reason why overworld games could not implement challenging survival mechanics in their exploration - the only thing they cannot in theory pull off that dungeons can is disorientation - it is that devs usually choose not to, and even remove stuff like no-fast-travel. That said, there is usually a conflict between the freedom of sandboxes and challenging survival mechanics, which is why I always thought Bethesda's best bet would be to focus on freedom and powergaming with Elder Scrolls - leaving them easy - and make Fallout into a franchise focused in hardcore overworld exploration with tough survival mechanics and limited options. But instead of that Skyrim and Oblivion removed a lot of the options that Morrowind gave you, while the Fallout games are still not very challenging to explore (though to be completely fair, I will admit that the Survival Mode in Fallout 4 was a step in the right direction).
As for "adventure hybrids", I don't know. Aren't Gold Box games and Jagged Alliance 2 tactics hybrids?
As I pointed out above, most RPGs borrow elements from other genres, even dungeon exploration is not the exclusive preserve of RPGs, Zelda and Metroid games have very elaborate mazes as well. Most RPGs are hybrids in some way or another, don't you think? Reactivity-RPGs are CYOA hybrids. My view on this is, so long as there is heavy character and party customization, and the game's design revolves heavily around that, it is an RPG. As Zed Duke of Banville pointed out above, combat and exploration might also be considered essential, but what you call adventure hybrids have all of those things too. Still, I think this is just a nitpick, I think we all agree that both Baldur's Gate and Wizardry I are RPGs.