>>1567012
Doesn't seem to be very accurate.
Apparently the "b" in "/b/ - Anime/Random" comes from the name of 2chan's secret (aka backroom) boards, which moot copied:
https://wiki.bibanon.org/4chan/History
So the term does actually have a previous history associated with 4chan. That might be what this guy is thinking of?

I've never heard of this before, and looking at the archives I don't see any evidence of people using the term to refer to 4chan's hidden boards:
https://desuarchive.org/_/search/text/backrooms/start/2015-11-07/end/2019-05-13/
(That date range is from when /trash/ was first added to the original /x/ backrooms post)
Looks like all the results are just people referring to real backrooms in places they worked or in the metaphorical sense of a "backroom deal".

I guess it's possible that some people may have once or twice used the term to refer to hidden boards. It just doesn't appear to be anywhere near as widespread as this guy thinks. And there's no world where the term "backrooms" was coined by 4chan users. It's a word that already existed. It seems pretty obvious to me that the writer of the original /x/ copypasta simply used that word in its original literal sense to mean the places in the back of reality. Why would an extra 4chan influence be necessary?