I’ve always understood 2 as 2 physically different media - i.e., copies in different folders or partitions of the same disk is not enough to protect against failure of that disk, but a copy on a different disk does. Ideally 2 physically different systems, so failure/fire in the primary system won’t corrupt/damage the backup.

Used to be that HDDs were expensive and using them as backup media would have been economically crazy, so most systems evolved backup media to be slower and cheaper. The main thing is that having /home/user/critical, /home/user/critical-backup, and /home/user/critical-backup2 satisfies 3 copies, but not 2 media.