I am unclear on what is meant by ROM. Don't most smartphones store their operating system on the same flash memory as user data? If it is all on one writable device, then ROM chips don't enter into this discussion.
The device contains memory that is only writable during ROM image flashing. The rest of the time this storage is not writable. So it is effectively ROM even if it isn't implemented by actual read only memory.
ROM refers to third party distributions of OSs for phones.
I can't find anything documenting the etymology, but I assume it's derived from rips of games used in emulation, which are in fact downloaded from "read-only memory".