It‘s not only about using the tpm to unlock the FDE, you should be able to do that on every distro with systemd-cryptenroll. The part that is new, is the the measuring of the systems integrity. It’s a way to ensure that the firmware has not been tampered with, the boot loader is the one that was installed and has not been replaced, that the kernel is exactly the one that comes from the distribution, that the kernel command line is the one that we expect, and that the initrd that is used does not contain any extra binary that we do not control.