You're correct, but only when speaking of the base models. Indeed, base M2 models can have slower disks than base M1s, accounting for model type. Apple has opted to use fewer NAND chips on the base M2's, which means less data can be written in parallel, so these are slower as a result.
Regarding the 2020 M1 Mac Mini specifically, while I haven't measured the disk speeds on my specific system,
this article from Tom's Hardware has this to say about the '20 Mac Mini with 256GB SSD:
My 14" has the 1TB drive, which has 4 NAND chips instead of 2 on the 512GB models. I benched it out of the box at around 6000/5000 read/write via BlackMagic. It's an absolutely speed demon by comparison! Feels much more comparable to the Gen 4 nVME drives I have in my x86 systems.