That doesn't matter. If you can't beat someone at their peak, it's a hallow victory.
Imagine if Frieza dropped back down to his first form when Goku went Super Saiyan for the first time, and then Goku demolishes him in a second. Would that prove Goku was stronger than Frieza?
And Luffy can fight top tiers in his base. That was already proved in the Kaido fight. For some reason though, Oda has this obsession with making Luffy go all out in the first round of his fights, and using a form that drains his stamina in minutes, instead of having Luffy fight normally and build up to using G4/G5 when it's more viable to use it. You can't beat a Top Tier that fast, no matter who it is, which is why when Oda has Luffy do this BS, we all know it's just to prolong things.
If you can accept that Luffy doing something stupid like burning through his own stamina and making himself weaker, which gives his opponent an easy victory, is indicative to that person being stronger than him, then more power to you.