The Overton window describes movement on a political axis by views held palatable to the public but does not reject the concept of objective definitions of “right” or “left.”
For example, the Overton Window would imply that there were right, left, and center Nazi party members. And there are, in the frame of the Fucking Nazis, but that doesn’t make a “left” Nazi an actual socialist.
Saying “Macron isn’t center” can be taken as an objective assessment of his politics. And it’s correct. He is not a centrist. Who don’t actually fucking exist anyways, but whatever.
So, basically, OP is saying you’re playing the fascists’ game when you let them define politics by relativism instead of objectivity. Bowing to the terminology of the Overton eventually leads to arguing about which “centrist Nazi” you find most palatable.