That’s a fairly common fingerprinting technique. A lot of sites use this and similar techniques (e.g. GPU fingerprinting, font fingerprinting, etc.) See https://developer.mozilla.org/en-US/docs/Glossary/Fingerprinting

Or try https://amiunique.org/ for a live illustration how that works.

Good thinking :)

To be clear, grub itself doesn’t mount your root partition. Grub provides the root partition device path or id to the kernel via kernel command line. Then kernel mounts the root partition early in its boot sequence with a limited set of options (typically read only).

Logically, the root partition should be mountable without fstab because otherwise it would be a chicken and egg problem :)

Usually once the userspace starts, an init script will remount the root partition with its normal options from fstab.

(It’s a bit more complicated if initramfs is involved, but the principle is the same).

Yep, they technically didn’t claim it was to make it more secure.