As far as I know ID Requests on Arqade aren't off topic as I asked this question almost a year ago and it was never closed, the link in the OP's question is dated back to 2 years ago. To my knowledge ID Requests must meet certain strict criteria otherwise they're closed, Arqade's I think is a screen shot, Anime and Manga has a Meta Question on it requiring 2 points or a picture, (my question originally didn't have one but I did link to the video and indicated the time, someone edited the image in for me).
As for if something is on-topic here I'd think of it like this. If the story contains one of these elements as a major/significant plot point it's on topic:
- A time setting in the future, in alternative timelines, or in a historical past that contradicts known facts of history or the archaeological record.
- A spatial setting or scenes in outer space (e.g. spaceflight), on other worlds, or on subterranean earth.
- Characters that include aliens, mutants, androids, or humanoid robots and other types of characters arising from a future human evolution.
- Futuristic or plausible technology such as ray guns, teleportation machines, and humanoid computers.
- Scientific principles that are new or that contradict accepted physical laws, for example time travel, wormholes, or faster-than-light travel or communication.
- New and different political or social systems, e.g. dystopian, post-scarcity, or post-apocalyptic.
- Paranormal abilities such as mind control, telepathy, telekinesis, and teleportation.
- Other universes or dimensions and travel between them.
Source
I'd also add anything which is seen as Magic as well regardless if it's actually a mystical force (Magecraft, True Magic in the Nasuverse), super advanced technology which is seen as magic by everyone else (Asgard, Goa'uld from Stargate) or a hybrid of the 2 (the Program like Magic Systems in Magical Girl Lyrical Nanoha) since it kinda is a merger of the 5th and 7th points
But then you have to ask yourself, where can I get a better answer? Practically almost every Anime/Manga/Light Novel/Visual Novel is sci fi whether it's using magical powers like in dragonball or weather it's just science like in Robotech, even Arqade accepts story-line questions like this one. In this case I'd do a search for existing tags on here, Arqade and Anime and Manga, see how many questions have been asked/answers and how detailed they are, which ever site has answers more to your liking is probably your best bet.
However it's a good idea to think about what you're trying to get out of the question ie. I asked this question here over Anime and Manga because I was after an ID of a Cthulhu Mythos creature, I knew from the references I give that Mary met something from the Cthulhu Mythos and figured I'd more likely get an answer here considering the number of questions on the Cthulhu Mythos and Lovecraft.
{Question about Anime}
TV and Movies -----------Ask On------------
/\ || ||
Ask On || || Ask On ||
{Question about TV or Movie} || || {Question about Sci Fi Story} ||
|| || ||
{Question about Story} || \/ {Question about Anime} \/
Aquade --------Ask On---------> Sci Fi --------Ask On---------> Anime and Manga
|| /\
|| ||
--------------------------------Ask On-------------------------------
{Question about JRPG Story}
^ above is what I'd like to think how ideally questions get put on a site when one thinks about where to ask, ofcause we still get Duplicate Cross-Site Questions so it doesn't work all that well.