> what's wrong with legitimately not knowing what e.g. the data structure will end up looking?

But that's not what the above comment said.

> Just let it run, check debugger/stdout/localhost page and adjust: "Oh, right, the entries are missing canonical IDs, but at the same time there are already all the comments in them, forgot they would be there

So you did have an expectation that the entries should have some canonical IDs, and anticipated/desired a certain specific behavior of the system.

Which is basically the meaning of "what will the output be?" when simplified for programming novices at university.