Sanji knows what an exoskeleton is ever since he was a kid. He just recalls everything for the users to know what he's about to do. That's the author's way of explaining his newfound powers. Sanji himself always knew a stronger body would accommodate hotter flames, which is the whole point of him training his Haki. It's his body and his flames, and he's been training to improve his body. It doesn't take Vega Punk to know exo-gives him the stronger body that he needs to host hotter flames.
Even if you what you say is true it still doesn't confirm anything
It's impossible to tell how strong a new form/attack will be until you actually use it
There's no way for Sanji to magically "sense" how strong his kicks will be after his new power up. He has to test it first
Luffy came out of Udon thinking ACoA was all he needed to take down Kaido and was proven wrong after he actually fought him
Zoro was hoping he could put Kaido on his ass with Asura (I know he was nerfed) and was proven wrong when he actually hit him
Sanji didn't have a chance to gauge his full strength till after he beat Queen