Xbox 360's 3 PPE Cores ran at 3.2 GHz, yet Microsoft were able to emulate them on the 1.75GHz Jaguar Cores of the XBox One, they had 6 and a bit cores available on Xbox One.
The PPEs used SMT, so each of the significantly smaller Jaguar Cores, with far slower per thread/core performance was able to achieve better performance compared to the XBox 360 versions of most of it's games.
Jaguar Cores are OoO Execution Units, they had no issue once Microsoft wrote the correct Emulation Software, along with adding the per game software wrapping emulation was achievable on the slower hardware of the Xbox One compared to the current gen AMD APU technology.
SPUs obviously add an additional factor that Sony has to tackle, but RPCS3 can run on older, weaker APUs than what PS5 has, first gen Zen, with half the cores and threads, plus a much slower GPU. PS5 also has the Tempest Engine that could assist in SPU emulation, since Cerny did say that there were similarities to the way that SPUs work on PS3.
Machine Learning within PS5's hardware and software stack could approximate PS3's processing intricacies to allow for Backwards Compatibility, really it's most likely just a matter of whether Sony wants to have some software engineers write or even repurpose open source software like RPCS3 to allow for BC to work for PS5 owners.