I agree Sanji likely won't go to the roof but him not being able to hurt Kaido and BM doesn't mean he can't be useful. His powerful kicks can get his opponents off balance as we've seen many times, even against Oars who is supposed to be reminiscent of Kaido to some degree.
Not to mention his invisibility would allow him to do it without them seeing him coming and it would create an opening for someone else to connect a powerful hit. Just imagine Sanji using Anti-Manner to get Kaido off balance, Zoro reopening Kaido's wound then Luffy landing the final blow.
Ok, now back to reality.