Unless of course you’re applying for an STCW endorsement, in which case it means Able Seafarer.
But they are. Legally, per the CFRs that allow you to hold that rank, Master is specifically listed under the Deck Officer section. Certainly in charge of the whole ship, no one would dispute that with any (every) Master who feels obliged to remind you as much. But just because the Captain is overall in charge of the Engineers doesn’t magically make them an Engine Officer too. They’re still a Deck Officer.