5e, to me, means material at least loosely compatible with other 5e material. I can use a 5e monster in any other 5e game, for example. I can run 5e adventures from one publisher with 5e characters built from the core books of a different publisher. Spells, magic items, monsters – these are all compatible with other 5e systems. Some material, like subclasses, species, races, feats, and other work might take some conversion work but are also generally 5e compatible.
There are some games built off of 5e that aren't really 5e. I put Shadowdark into this category. It clearly has 5e in its bones but it isn't really compatible in any reasonable way.