See, the problem I see with that is that while a game should be approachable by itself and critics do have to review other things other than the plot... the plot still is a pretty important part.
I mean, let's not kid ourselves... I seriously doubt KH III will take its time to make Xehanort and all the other vessels understandable and approachable to newcomers, or TAV or RAX relatable to whoever didn't know them. Or even justifying Sora's bond with his Keyblade or Donald and Goofy. This is a game that has been foretold since forever to be the conclusion to every other title, so every other title will play its part in being necessary to get what this ending part is about.
KH III will be like Star Wars VI. Still a "good guys vs bad guys", but fat chance if you hoped to get why is Luke skilled with the Force or who exactly is Darth Vader from just that movie.
Now, that being said, anyone is free to play anything regardless. Gameplay will be still its own thing, so will minigames, exploration and stuff. And of course one can still get the main gist of "go to Disney worlds and beat up baddies", and let's be honest, there are many "fans" who actually still have that as their main knowledge of previous games.
We get "Aren't Roxas and Ventus practically the same person?" every other week on these forums, and if you ask me you must've played some hardcore blindfolded DDD with audio muted to not get that they're different.
AngryJoe can play KH III blind if he so chooses and that's his right. I'm familiar enough with his style to know deep story analysis isn't really his thing anyway.
That being said, I at least understand where the concern is coming from (concern. Not angry fan mob harassment). Bad rep and misinformation are KH's greatest enemies, and if a relatively well-known youtuber plays KH III without the other ones... and God forbids he will say stuff like "the plot is too complicated!" in his review... people will keep riding that train and the old memes won't die.
I can just avoid AJ's video, but hundreds of thoudands of people won't. And they might become that vocal percentage of people we see and resent during KH news that just bash on the franchise.
About his persona... he like many others mixes "fake" anger with real one (kinda like Cinemasins, which always say they're doing a parody of an overly nitpicky viewer but you can sorta get when they ACTUALLY hate a movie)
Like, I was speechless when after the Microsoft conference at E3 we got that special and they talked like the last time HJO voiced Sora was KH II, and kept talking about how many years have passed between KH II and KH III.
This is how you keep the misinformation going.
So, to summarize: while I still think KH III does require more knowledge of previous games than the average, it is never acceptable to lash out at one dude for not wanting to play a good decade of videogames just to play another videogame, but at the root there is a sentiment I at least empathyze with.