Except that the pseudodragon can only make that bond with a spellcaster
That's not in opposition to anything which I have said.
And the variant at the back of the MM that discusses an NPC spellcaster getting a pseudodragon familiar specifies they must be able to cast find familiar.
That side-bar has zero interaction with the other side-bars, and demonstrates nothing except that NPCs are not held to the same rules that PCs are - most strongly evident in mentioning "...or some other Tiny monster, such as a crawling claw..." which is not available as a familiar for PCs.
That side-bar also doesn't actually state a requirement for the NPC to be able to cast
find familiar in order to have a familiar - it's mention of
find familiar is that any spellcaster that knows it is likely to have a familiar. The two are significantly different.
Nothing says that an NPC can't have met a pseudodragon, imp, or quasit, and had that creature offer up the service as familiar mentioned in those creatures' respective variant side-bars.
Almost everyone seems to ignore at least one of the sources of information, probably because it is messy trying to fit all of them together.
I ignore the sources of information circumstantially, such as when one source is written without requiring any interaction with another source.
So I don't ignore the
find familiar spell when working out how the Pact of the Chain feature works because it specifically calls out interaction with the spell, but I do ignore a side-bar about NPCs having familiars when not dealing with an NPC, or the variant familiar side-bar when it is the unrelated
find familiar spell, rather than the creature's choice, resulting in the creature becoming a familiar.