Dictionary says flat, plain, may be it can be coldly?
This ひら doesn't mean 平(flat, plain) but ひらり.
Dictionaries say ひらり means
ひらっと is almost the same as ひらりと, which means "lightly", "nimbly".
「ひらっと」 is not a very common onomatopoeic adverb to use in the context of passing by someone. It would, however, mean "casually", "quickly", etc.
More commonly, you would encounter 「さっと」、「さーっと」、「すっと」、「すーっと」、「ふらっと」, etc.
Not sure what dictionary you use, but 「ひらっと」 just could not mean "flat or "plain" in the context. You would not pass by someone "flat(ly)" or "plain(ly)", would you?
"Coldly" sounds off to me as well for the context.