You're right about the ROM, but parent is talking about something else: the built-in bootable system that came in ROM on the Classics. Cmd-opt-x-o at startup would boot you straight into 6.0.3, at the cost of RAM (it made a RAM disk to run from).
I used to ROM-boot my Classic back in the day, it was nice to have a built-in recovery volume. These days if you use a Mac Rom-inator II[1] it supports booting System 7 from ROM, and it'll even create a RAM disk the system boot from, making the system ephemeral but modifiable. I have one in my SE/30 and it's been helpful to have that built-in recovery volume - no floppies needed.
You're right about the ROM, but parent is talking about something else: the built-in bootable system that came in ROM on the Classics. Cmd-opt-x-o at startup would boot you straight into 6.0.3, at the cost of RAM (it made a RAM disk to run from).
https://lowendmac.com/1990/mac-classic/