While there are issues concerning that these comments are promoting another site, and that these comments don't seem to realize that site scopes can overlap, there's a more serious issue going on. It has to do with how, exactly, Literature.SE is tending toward working, and why that's actually quite different than how SFF.SE works.
Asking a question on Literature Stack Exchange opens you up to a categorically different type of answer and perspective than Sci-Fi Stack Exchange does. Making these comments sort of assumes that the intent of the author was to get a specific kind of answer - i.e., the factual, "the author says X" or "text says X" answer that's usually a baseline requirement for posting on SFF.SE.
There's nothing intrinsically wrong with the kind of answer SFF provides, don't get me wrong - but answers on Literature have shown (with very good reason) that broader perspectives, and sometimes wholly subjective ones, are important to answers here, too. We have to start with the assumption that, if a question was asked here, it was asked here intentionally, because the OP was looking for a potentially different kind of answer than the kinds that SFF.SE can provide.
I'll grant that some questions make quite explicit that the sort of answer they're looking for here is the same as could be found on SFF.SE. When that happens, see the above. But, importantly, this isn't always true.