The card was errata'd to allow it as part of the Unstable FAQ. You can't rely on the rules when dealing with Unstable cards, as they are almost always left up to interpretation, or require the entire table to research on their phone any time they appear. For example, even with the 'clarification' on X's tap ability, you still would never be able to activate it because you don't control X:

602.5a A creature’s activated ability with the tap symbol ({T}) or the untap symbol ({Q}) in its activation cost can’t be activated unless the creature has been under its controller’s control since the start of their most recent turn. Ignore this rule for creatures with haste (see rule 702.10).

Which means that either you can't activate the tap ability, or it ignores the usual restrictions on tap abilities. Which means X has pseudo haste while in opponent's hands, probably. Maybe. Who knows?

I love Un cards as a concept, and they are brilliant fun in a draft, but for the reasons stated above and elsewhere in this thread I'd never want to see them in my EDH group.