Well of course VoLTE is more secure
I can't hang a device off a carrier's VoLTE network like I can off a wifi network! I imagine there may be worse security failings in VoLTE, and much of its improved security relies on the carrier's network being much harder to access.
This is like saying it is easier to pickpocket someone when they are walking on a public sidewalk than when they are walking down the halls of the Pentagon.