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".
Re-read the quote I have in my comment and remove the word 'literal'. Does that sentence have more, less, or the same meaning / power to you?
With 'literal': don't stand by the wall
> There’s a moment in your book that I love, when you give advice to awkward people looking for social shortcuts, and you tell them not to be literal wallflowers.
and
> There’s a moment in your book that I love, when you give advice to awkward people looking for social shortcuts, and you tell them not to be wallflowers.
alternative
There is a non-trivial difference in connotation between the two sentences.
In vernacular conversation "wallflower" can be used as a term to connote (strong) shyness or to indicate generalized presence on the periphery of an event. In the sentence from the article, "literal" helps to shift emphasis away from the "shy" connotation. Without it, or some other way to emphasize connotation, the sentence is easier to misinterpret as follows:
"..., and you tell them not to be too shy."