You can define a one-to-one mapping between the sets {1 2} and {3 4}, but I don't think anyone would say they are equal.

You’re thinking of isomorphic, not equal.

They meant "their cardinalities are equal". It's honestly an easy mistake to make, especially if typing on a small screen. Or especially if having a discussion where sizes of infinity are already being discussed.

equal in cardinality, yes, thanks.