Okay, this is absolutely an overly pedantic question for what is most likely meant to be a lighthearted challenge, but out of curiosity, what would you describe as a "sentence" in this context?

Over on Code Golf Codidact I'd probably overthink it and include a possibly excessively precise definition, but here I feel that the challenge benefits from being open to interpretation.

Go ahead and interpret it as restrictively or as leniently as you see fit (you're even free to post more than one answer if you want to try out different interpretations).

I have a few ideas of what it could mean, but I'm deliberately not sharing them. I have a suspicion that the challenge author mentioning a particular interpretation of a deliberately ambiguous phrase might make people less likely to find other meanings that I haven't thought of.

I'm curious to see what different people come up with.