The irony is that they themselves are the biggest example of it...
There is no place that PoE is more hated than the codex. Every time PoE is mentioned there is a legion of people spewing vitriol for the game and Obsidian of course. People here don't claim it is "meh", they claim it killed their firstborn.
Why would they do that if the theory is "that the game was mediocre so we didn't bother leaving reviews for it". Yeah, you certainly keep silent about your hatred for the game, and it shows, especially on the Codex...
And it makes sense: People expected to relive their childhoods through PoE, to feel the same feelings the BG saga made them feel in the 90s when they were still young pure souls and not dead inside 30-40somethings. But this was impossible no matter how good PoE was. So after they felt "meh" about PoE, they decided to hate it eternally.
And since Chris Avellone left Obsidian, the hatred multiplied here because we all know rpgcodex is the unoffical Chris Avellone "i beg you please let me lick his balls" fanclub.
To an objective critic, PoE was a really good game. It had many faults at release and it got vastly improved with patches and DLCs. But still deserved its metacritic score. I mean, what competition there was? Outside of Witcher 3 and perhaps Metal Gear, Fallout 4 and Rise of the Tomb Raider what other game was actually good that year? I don't mean masterpiece, just good... Was Age of Decadence released in 2015 IIRC? Add that to the list.
I feel that sometimes people lack context. This generation of games is not exactly chokeful of quality content... We get a metric crapton of games but most of them are shitty shit full of shit. It is nothing like the 90s when interesting quality games were releasing left and right both on PC and on consoles and still Baldur's Gate was king of the hill and considered AAA by then standards.
Yet people hate on PoE because it didn't make them feel like BG made them feel 20 years ago. Then they create circle jerks against the game and people won't buy the sequel...
Divinity Original Sin got away with it because no one expected Baldur's gate. It came out of the blue, had no previous expectations, so people judged it closer to what it was rather than what they wanted it to be.