tl;dr Crypto cannot be a collective delusion because that is a different type of object. (Crypto can still suck though)

What you've described a collective fictions are variously called: Quasi-Abstract-Objects, Institutional Facts and these exist in a social ontology. Yes, this ontology is not mind independent (ie is subjective) but we can still stay objective things about them.
That makes crypto a collective fiction at least while people continue to treat it that way. There's no inherently special physical properties about the 1s and 0s that encode crypto that distinguishes crypto from a digital porn stash, but we have given crypto meaning and enforce that with programming code.
The delusion examples point to things that could never exist. Frodo Baggins from LOTR is impossible within our known spatio-temporal reality. However, the character Frodo Baggins can exist as a delusion for us.
Crypto (and even fiat) is not the same category of things. We can't magic Frodo into existence by all agreeing that Frodo exists, but we do exactly that with stories about Frodo and we do the same with crypto.