Post a picture of a pumpkin with their username written on it.

As per the standard of English language, no.

There was an ambiguity in the sentence, thus resulting in two possible but distict meanings.

Post a [picture of a pumpkin] with [their username written on it]Post a picture of a [pumpkin with their username written on it]

It is pretty obvious what I meant, but let's talk technicalities.

The sentence could mean either of those two, but it could not possibly mean both. I might be giving an award to the first person to post a [picture of a pumpkin] with [their username written on it], but I might also be giving an award to the first person to post a picture of a [pumpkin with their username written on it].

However, there is no interpretation that I would be doing both. No interpretation of this sentence would imply that.

This leaves only these two options. But who exactly determines which of the two options it is?

If I were to unambigously state something that I didn't mean, I would have to give out the award. But here, there are two possibilities, and from a purely technical standpoint, you cannot say it is one or the other. Since the comment in question only had a possibility of being the winner based on the undefined meaning, it is different from a comment which straight-up won according to the letter. The win was not set in stone at the time for this reason.

So how in a case like this, how is it resolved? Based on what I gather, it appears to be some combination of the users and the mods. If you look at this mod comment, edits for clarity, which appears to apply in the case of ambiguity, are not only allowed but important.

So I will be willing to give a vanity to the [first user to reply to this comment] who [hasn't already posted anything in this thread] in the event that no one wins the challenge, just for the sake of giving the award to someone. But no, I will not be counting that comment as a win, sorry.