Frankly, I don't see any rationale for such a purported lie...why do you think there is?

from the linked ProPublica article this is just a rehash of old news...possibly to extract the last drop of 'Afghanistan' criticism...but who knows.

Link: https://www.propublica.org/article/report-u-s-marines-returned-fire-after-suicide-bombing-but-no-enemies-were-shooting-at-them