Mercedes technical director James Allison suggested at the Canadian Grand Prix that Red Bull had perhaps made its RB20 slower with recent updates it had introduced.
"It does look as if their upgrade was a downgrade," explained Allison. "So, fingers crossed that would really mess them up.
"That [an upgrade not working] makes life hard, because the moment you stop trusting your tools, you have to backtrack, and you lose loads of time. Time is your biggest friend, losing it is your worst enemy."
But with Red Bull having come out on top in the Canadian GP, as Mercedes' George Russell came home third after starting on pole position, Horner could not help himself with a swift quip about Allison's remarks.
"Even with our downgrade, we managed to beat their upgrade," said Horner. "It was a rewarding race to win."
Max Verstappen's triumph in Montreal came on a weekend that had been expected to be a challenging one for Red Bull, with its kerb riding weakness being exposed.
So having managed to pull off a win like at Imola that could have gone to someone else, Horner said he was relishing the prospect of now heading to tracks that were more about ultimate aerodynamic performance.
"The last couple of races have been more choppy waters for us, but we have still managed to win two out of the last three races," he said.
"We had a pole in Imola, we managed to match the pole time here with a car that both drivers are feeling its deficiencies. So there is a lot of focus on that to see if we can improve that.
"We know there are circuits later in the year like Singapore where it [ride issues] could be a factor. But we really expect Ferrari, McLaren - and Mercedes who came into that window this weekend - to be competitive at every circuit.
"So I think despite having won six of the nine races - we are going to have to be top of our game to keep eking out a gap."
But despite Mercedes being a threat in Canada, Horner felt it was far too early to suggest that the German manufacturer was properly back in the game.
"Mercedes have always gone well on green circuits and there are no real high-speed corners here," he said.
"So with the asphalt change, it played possibly to their strengths. So let's see over the next three tracks if it is genuine form or if it's a one-off."
