The word "literal" here is slightly more complicated than in the usual commonplace misuse of the word.
The author is trying to distinguish between "wallflower" the "socially awkward person" and "wallflower" the "person who stands near a wall". They are using a slightly more literal definition, but not the most literal (i.e. the plant).
The justification is that "being near the wall" =/= "being in a high-traffic area".
The author is trying to distinguish between "wallflower" the "socially awkward person" and "wallflower" the "person who stands near a wall". They are using a slightly more literal definition, but not the most literal (i.e. the plant).
The justification is that "being near the wall" =/= "being in a high-traffic area".