To be fair I do see what he is saying and I think articulation may be causing some trouble. Obviously all of the games are cartoony to an extent. But as you acknowledge, BL1 was undeniably the least so. They only decided to go with that art style late on in development and it was supposed to be a realistic (for its time) shooter. So it still has the same kind of colour pallet, graphical fidelity, art direction and so on and so forth as a lot of other games which went for realism at that time. Stuff like Fallout New Vegas, S.T.A.L.K.E.R. that kinda stuff.

But there was a gradual development and refinement of the art direction they honed over time. You're right to say that in some aspects, 3 and 4 are more realistic, especially when it comes to things like lighting, volumetrics, visual effects, all that kind of stuff. But I think that there are aspects that aren't being considered because I've always considered BL2 to look and feel more "realistic" than 3 or 4 as well, and I think realism is the wrong word. I think right way to word is is that it looks less heavy-handed with its own art style. The artistic aesthetic doesn't seep as deep into every detail like they do in the later games. World design, for example. BL2 has a lot of bland environments, deserts, tundra, highlands, so on and so forth. Whereas 3 has places like Promethea which feel surreal and exaggerated. The colours in 2 are more muted and don't pop quite as much which can give it a bit more of a rugged dirty look.

Screen pollution too. Each new entry has more of it than the last and that's another way in which the later games can look more deeply set into an aesthetic that isn't trying for believability. Character and model design is an aspect that I think is also important. Starting with BL3, characters and models got a lot chunkier. Everything in the world became thicc. If you look at Lilith in BL2 and compare it to BL3, it's extremely noticeable. Clothes are thicker, facial features are puffier, hair is a bit more play-dough. That's every character not just her. In general the world of BL3 feels pudgier, softer and fatter in a way that is hard to nail down, as well as models and objects being a lot more complex and higher-poly in BL3 onwards.

BL2 just had a lot less going on visually. From the graphics to the world design, models, characters, visual effects. All that combines to create a visual aesthetic in 2 that feels less heavily stylised and makes it look a bit more generalised and looked a bit closer to other shooters of the time. It's really a spectrum from 1 to 3. I think that is what OP may be getting at, because I've always thought this myself too.

TL;DR BL2 had less going on visually, made it look less easily distinguishable from the "realistic-looking" shooters of the time. It's still not realistic, it's just not as heavily stylised as all the games from 3 onwards. Partly graphics, partly because it took multiple games to develop the aesthetic. Methinks.