The PS5, like the 4, is constantly recording a set length of your recent gameplay and dumping it second by second past that threshold. It's how the Share functionality works. On PS4 if you had a PS camera or a connected headset there was the option to record microphone audio in Share footage as well, it's just that now everyone has a microphone. But that footage is constantly being erased unless you choose to save it.
I'm 95% sure that the include mic audio setting was disabled in the start and I had to enable it (I like it), so I think this person must have enabled in the settings but not realized what it meant?
Also, you have the option to disable automatic trophy clips like this one. So for comfort you could have no trophy clips, mic audio capture disabled, and then mute the mic on the controller.