While I agree with the majority of you comment and take on superfans in general, I think it's important to note that a large amount of those "superfans" for Sanders are actually astroturfing efforts.
I don't have sources handy, so take it with a grain of salt, but it seems pretty clear to me that on Twitter and Reddit those "superfans" are just trying to promote the idea that voters shouldn't vote for Biden as a way to remove their vote in the general.
Again, I agree with what you said here, just want to point out the astroturfing happening.