(@BustedPancake@lemmy.world's use of mkisofs does the same thing because they copy the files on the disk rather than the whole disk. But you don't need makemkv. You should be able to use any method of copying the files and Linux should use libdvdcss to decrypt them.).
“deep magic”
Linux trys to treat devices like files. If you ran xxd /dev/cdrom, you would see every bit on the disk (not just those of the files, but those in the free space as well) in order from the first to the last (converted to base-16 in what is called a hexdump). Not that you need to see this, but your video player does. The “DRM cracking” is actually a feature of libdvdcss that makes it possible for the system to treat the disk this way. dd is just a general copying command and if Stack Exchange is to be believed, it isn't necessarily the best option (https://unix.stackexchange.com/questions/12532/dd-vs-cat-is-dd-still-relevant-these-days). But it probably is necessary for the linked guide to work because it has dd truncate the file.