Yes, I agree, except in the case that they split their finances. If combined finances, both would need to be able to ER (and one not because they like their job) for the one stopping work to be "retired" and not just "quit."
Even under MMM's definition, FI is a prerequisite for "retired."
If they have split finances, and the one is FI, and the other not, that's fine, as long as the ER'd one is living off their stache, not off the other's work.
(I've always thought split finances in a marriage is weird, but it works for some.)