Lemmy doesn’t assume anything. Sorting is done by votes or activity from all communities; the intent is for the user to subscribe to communities to curate their feed. But blocking works too.
I don’t know precisely how Reddit works now, but it used to be like that as well. There was no interest algorithm, no suggestions, other than a bit of vote count nonlinearity. It just had the quirk of defaulting to a few select subs in “hot,” instead of “all” like most Lemmy/Piefed instances do.