Eirikr70 Well in a sense they refer to the same thing, as it seems 'custom ROM' is a 'metaphor' that's 'historically' evolved , in a sense, to refer to it.
It's like dialing a number, people still 'dial' numbers on their phones, while there haven't been physical dials for decades. Technically the term isn't correct anymore.
While i'm not sure of the origin of 'custom ROM' and it evolving into a linguistic term to describe a custom OS, technically it doesn't make sense (anymore?). GrapheneOS wants to, rightfully so, break away from that innaccurate term, and wants it's project to be described correctly as to what it essentially is, an OS. I also believe it doesn't want to be 'associated' in any way with other OS'es in that 'custom ROM' group, as it is in a completely different ball park. For example on xda forums, mostly a 'custom ROM' is kinda hacked togheter by a single hobbyist, while GrapheneOS has proven, again and again, to actually be a full fledged high quality OS. They don't refer to OneUI, OxygenOS etc as custom ROM's, while in theory they could aswell since it's also a variant build on top of AOSP, but they don't.
Custom ROM's indirectly are associated with hobbyist projects, which obviously isn't the case for GrapheneOS. GrapheneOS, for it's purpose, is the gold standard even exceeding OneUI, OxygenOS, etc.