Well, a disk image can either be sector-based or file-based and the disk image can either be created from an external OS or from within the OS. What you call a drive snapshot is taken from within the OS. The key phrase on the page you linked to is “While Running Windows.” So that is a completely different thing and of course it is a standard procedure.
But the question remains if it is a standard procedure to boot from an external OS to create a DR backup from a BitLocker-encrypted drive. I would say no.