This wasn't that unusual though? The PS2 contains the PS1's CPU and uses it for I/O when it isn't being used for backwards compatibility with PS1 games. The PS3

This wasn't that unusual though? The PS2 contains the PS1's CPU and uses it for I/O when it isn't being used for backwards compatibility with PS1 games. The PS3 contained the CPU/GPU of the PS2 in early versions for PS2 BC (https://www.psdevwiki.com/ps3/PS2_Compatibility https://www.psdevwiki.com/ps3/PS2_Compatibility). It's only with later PS3 iterations that the PS2 hardware was removed in favour of software emulation for BC.