There’s a lot that looks good on the first few layers of Unbreakable that don’t hold up when you really look at it. The reveal of his super strength had a great feel, but doesn’t hold up because the either David is terrible at strength training, or he didn’t get his powers until the train accident, and we know he actually had them when he was a teenager because of a different story beat. And he’s probably not terrible at strength training because he was on track to be a professional football player. So it really doesn’t work.
Forgiving that, though, I felt the text was trying to say something in the neighbourhood of what you are saying: that David’s ability to lift or grip is probably only limited by what he believes he is capable of. The whole “What else do we have” – “That’s everything” exchange, and he still lifts it, with about the same level as effort as the first attempt seems to reinforce that. And it also nicely dovetails with the film’s text that Elijah and David are complementary figures. Elijah is powered by complete self confidence, limited only by his breakable body. David is powered by a nearly-unbreakable body, and limited by almost non-existent self confidence.
Maybe I’m overly defensive about my own enjoyment of comics and genre, but I also feel like the film is not bullish on comics. The stark contrast of the (in) action scene to the vibrant, consequence-free action of the comics presented in the film, and the fact that it’s the villain acting as the voice of the value of fantasy tells me that (in what had to be willful ignorance of the exploration of grittier, more grounded super hero tales that was going on well before the film was shot) that the director was making a point of the childishness of indulging in fantasy. The best parts of the film are about ordinary life, and I don’t know if that was entirely a mistake.
Maybe it was? M. Night Shyamalan has repeatedly shown an incredible ability to whiff the second half of his films, so who knows.
I did love the deliberately uncomfortable messiness of that action scene though, despite the lazy creepiness of its setup.