Paul Farquhar
Legend
So does fireball. So what?They're using the spellcasting mechanic.
Yes, yes they do. Specifically, Find Familiar does:Notably, the connection is that they neither of them really changes the creature
"the familiar has the statistics of the chosen form, though it is a celestial, fey, or fiend (your choice)"
A familiar summoned by a Find Familiar spell uses the (modified) stat block of the form it takes, but it is not actually that creature.
Note that there is no "Variant Imp" in the Monster Manual. There is a "variant familiar" in the Monster Manual. The variation being, the familiar is acquired by the master entering into a contract, not by casting Find Familiar. If you get a familiar in this way it is different to a familiar gained by casting Find Familiar. This contract gives the imp a telepathic link with it's master, and the shared magic resistance ability. The shared resistance is gained from the contract, not the imp, which is a perfectly ordinary imp.