cdflasdkesalkjfkdfkjsdajfd This is also applicable to Ubuntu, debian, etc that are versions of Unix/Linux but not specific Operating Systems.
But people don't typically refer to their "Ubuntu ROM" or their "Debian ROM".
Many years ago phones had a single image that could more or less plausibly be referred to as a "ROM image". But if you flash GrapheneOS from the command line you will see that it is made up of several images. And, like Debian or Ubuntu, the OS running on devices is backed up by a server infrastructure that is part of the OS and definitely has nothing to do with "ROM".
When automobiles came out, people referred to them as "horseless carriages", which in a sense they were, but only if squinting really hard.
Calling GrapheneOS a "ROM" is like calling a Bentley a "horseless carriage", and also like calling Ubuntu a "ROM".