The Saints kept the Premier League champions to just one shot on target as they held Pep Guardiola’s side to a 0-0 draw at the Etihad.
The day could have gone even better had VAR not overturned Kyle Walker’s red card and the penalty after the Manchester City defender bundled Armstrong inside the penalty box.
The Saints striker insists that he felt that he was fouled and the penalty decision should have stood.
He indicated that on another day it could have been better but believes getting a point at Manchester City is still a big result.
Armstrong also highlighted Southampton’s ability to harry the Manchester City players and create the chances at the other end.
The Saints striker told the BBC: “I thought it was a penalty, but VAR can see it better than us.
“It is one of those days, we got the point and it is a massive point coming here.
“We’ve just got to move on.
“I thought we did well today; we created chances, but to get a point away at Man City is brilliant.
“We’ve got to take the positives and move on.
“We pressed high and showed them we weren’t afraid.
“The gaffer wanted us to create chances that way.”
Southampton are yet to win a game this season but have managed to hold teams such as Manchester United, Manchester City and West Ham to a draw.