The data is encrypted with the key stored in the T2 chip (if it's still called that) so you couldn't even just solder it to another board.

Shouldn't Apple be able to recover the data if T2 is intact? It's possible that it's not worth to them, but I'd think it's doable.

It's of course technically doable - the laptop itself is a device that talks to the drive via T2. But it's not something Apple wants to do. This was an issue for years already before M1 was introduced.

Is this the same as Filevault or is it something different?

Similar, but done in secure hardware in a T2 embedded inside the M1. The complete I/O path is protected.

You have to backup more regularly than every two months...

And Apple replaces the entire motherboard if a component on it fails. The agreement that you may lose data has been around awhile and it's just boilerplate. When mucking around on a board they aren't going to guarantee anything - and I don't know if third party repair shops would do that either. There is always the chance someone screws up and breaks something.

Replacing the motherboard shouldn't mean losing your data. But it does if you own an Apple.