I will admit that I am not fully aware of how the current system is setup. But from what I am getting out of the linked article and a few other sites, it seems that this is just replacing the current Ext4 system partitions that are already read-only. Which seems to have been a thing starting with devices that shipped with Android 10. I am not currently messing around with custom ROMs, so I am not up on the scene these days. So again, I am not coming at this from a great in-depth knowledge and could be misunderstanding things.
I will admit that I am not fully aware of how the current system is setup. But from what I am getting out of the linked article and a few other sites, it seems that this is just replacing the current Ext4 system partitions that are already read-only. Which seems to have been a thing starting with devices that shipped with Android 10. I am not currently messing around with custom ROMs, so I am not up on the scene these days. So again, I am not coming at this from a great in-depth knowledge and could be misunderstanding things.