[niam]

I feel like the word unambiguously describes exactly what it is, which is all I can really ask for from a word.

"Lesson" by itself might connote a more concrete transmission of knowledge (like a school lesson). Which is a meaningful distinction if the goal of the article is merely to muse about lessons they've learned rather than imply that this is a lesson from the writers to the audience. "Lesson learned" could imply the same thing, but is longer to say ¯\_(ツ)_/¯

I get what the comments here are saying about it sounding corporate, but I think this is a unique situation where this word actually makes sense.