His strongest mode is IJ??? He has multiple moves inside IJ lmao
His point is valid though; IJ isn't like G5, it's more like comparing base Luffy to G2 Luffy.
IJ doesn't give Sanji new capabilities, it's literally just a stronger version of DJ. It's his strongest mode. Every DJ attack can be replicated with IJ and it'll be stronger, just like every base Luffy attack can be replicated by G2 and it'll be stronger.
And Sanji doesn't have additional multipliers. For example, ACoC G2/3 Luffy > ACoC-less G4 Luffy. Furthermore, G5 is less of a mode and more of a transformation, so low-end G5 attacks are barely above base Luffy's AP. Whereas G4 - a mode - is vastly superior to base Luffy to where even its low-end attacks are up there.
Sanji using IJ means he isn't holding back. He may have stronger attacks in IJ, sure, but this is him going all-out. It's the same as if Zoro were using KOH or Luffy was using ACoC + G4/G5.