The Buddies twice took the lead through Eamonn Brophy and Jamie McGrath, but both times they were pegged back thanks to a Joe Shaughnessy own goal and a strike from substitute Jason Cummings.
The newly-promoted home side finished the game with ten men after Max Anderson received a straight red card with just over 20 minutes to go.
Despite that, Dundee held on to secure a draw which McPake reckoned was even more notable as he had a host of players self-isolating all week due to a Covid-19 case at the club.
The Dens boss said: "I'm extremely proud of my players.
"We had 10 players out altogether, two who didn't train all week and played today then another who came on who didn't train all week. To a man, they were outstanding."
While he was delighted with his players, McPake was less pleased with referee Craig Napier and his decision to award the penalty when Jason Cummings challenged Jay Henderson.
McPake said: "I've seen it, I've seen the angle that everyone will see and I've seen the angle from behind the goal. It's not a penalty.
"I'm not here to bash officials. I give them credit when credit is due.
"I don't want the talking points to be a penalty and a red card as I had 14 players who to a man were outstanding."
Dundee's day was blighted by a first-half injury to striker Danny Mullen, with McPake adding: "The injury is bad, it's his fibula and he's at Ninewells Hospital just now."
St Mirren boss Jim Goodwin admitted he was disappointed not to have taken all three points.
He said: "It was a decent game. There wasn't a great deal between the teams in the first half.
"When we have the extra man, you would expect the game to play out the way it did.
"I'm disappointed we didn't win but coming away to a promoted club on the opening day, when there is excitement and buzz, we should be relatively pleased with the point."
Goodwin sympathised with McPake over the penalty but still insisted it should have been given.
He added: "The game has gone soft.
"James told me he was disappointed and if the shoe was on the other foot, I'd have probably felt the same.
"But when I watched it back, there is a coming together and in the modern game, it is a penalty."