What's the difference between DFU and iTunes restore in iPhone?
DFU writes the firmware and then the OS. 
Restore writes the OS only. 


My guess is the misnomer is firmware like carrier updates and modem scripts will be layered on after either OS restore. Also, these names originated when the iOS devices were far simpler. Secure enclave, secure boot chain, forward secrecy and encryption at rest make the update procedure far more uniform. Maybe that's also what GSX is alluding to - you don't really write all the firmware anymore on current / modern iOS devices.