The Celtic midfielder admits he should have scored.
Brendan Rodgers's side picked up their first trophy of the season by cruising past the Dons en route to lifting the cup.
Tom Rogic and James Forrest fired Celtic into a comfortable 2-0 half-time lead, before a Moussa Dembele penalty on 64 minutes wrapped up the victory.
The 24-year-old, who played the full 90 minutes at Hampden Park, should have added a third before the break but he scuffed an effort wide from six yards.
And although Armstrong concedes that he perhaps should have scored, the midfielder is happy enough with the winner's medal.
“I should have scored,” he told The GET. “Obviously, I had a lot of time and I took an extra touch. It was a split-second decision with someone rushing at me and I tried to tuck it into the opposite corner.
“It went wide and it would have been nice to add a goal, but it was about winning the cup and I was just happy to end the day with a winners medal in my pocket.”
Celtic could and definitely should win at least a double this season, with Rodgers's side sailing to a sixth successive Scottish Premiership title, but they would like to add the Scottish Cup for a rare treble, too.