This is because the author didn't delete the entire volume group. You have to delete the whole volume group, i.e. erase the entire drive and it's partitions to reinstall OSX.
This isn't new with the M1 Macs. This has been a thing since they introduced APFS. When they went into disk utility after the cli - Then they could delete everything and kick off the OSX Install.
If the APFS Volume Group still exists the installer will try to unencrypt and work with it. So.. In short, just make sure you delete the volume group. There's a specific button for it. I just did this yesterday on a T2 Mac.
This isn't new with the M1 Macs. This has been a thing since they introduced APFS. When they went into disk utility after the cli - Then they could delete everything and kick off the OSX Install.
If the APFS Volume Group still exists the installer will try to unencrypt and work with it. So.. In short, just make sure you delete the volume group. There's a specific button for it. I just did this yesterday on a T2 Mac.