A tech YouTuber has conducted a test to prove that PS5 can emulate PS3 games, and has disputed widespread claims that PS3 emulation is a near impossible endeavor due to architectural and hardware challenges
There's a huge difference between "I can hack this together for some small subset of games" and a company putting their name on it for the games that leveraged the hardware completely (most of the ones people care about, because Sony is good at helping their studios get the most out of the hardware).
They do (or should) have extra information to facilitate patching issues, but there are very real and very obvious reasons that they can't just flip a switch.