You are right that the poster's intended argument seems to be that Lewis is a bad writer, in the sense of "wrote harmful things" rather than "is unskilled." The problem is that the poster's listed points boil down to, "I like female characters who get into physical fights and kill stuff! People shouldn't write female characters who stay out of battles," using Susan as the go-to example, thus implying that book!Susan is inferior to her movie counterpart. And that's stupid. Obviously people shouldn't ONLY write female characters who stay out of battles, but the reverse is equally true... and in fact Lewis did write female characters who fight in battles. Susan simply isn't one of them, and shouldn't need to be.

I disregard the points about Susan's subplot in TLB because the poster misinterpreted it -- the reason Susan doesn't get back to Narnia is because NONE of the Pevensies are allowed back to Narnia; the reason she doesn't go to Aslan's country is because she doesn't die -- and because, as previously stated, the only reason the movies didn't also do that plot is that they ran out of funding to get there, thus making the comparison inherently flawed.