If he can use FBH....then logically he can use a HM IJ.
And its not like FBH would be that difficult when people like Pica can do it.
Huh? Completely different stuff, though. Haki is haki. It's different from using a move from one mode while using some other mode.
IJ is its own mode with its own different pyre and likely its own different requirements/energy/limtations, etc etc.
Assuming Sanji can whip out HM in it is about as valid as assuming Zoro can whip out three times bigger Tatsumaki or something in Asura. Cool if happens and possibly sensible, but baseless.
I thought people assumed All DJ abilities = IJ abilities now because they thought IJ is just DJ with sparks lol. Oda emphasized even more on how they're completely different modes, even aesthetically.