This is very wise and careful : I won’t take the risk of using commands (…) that I don’t understand perfectly.
What’s proposed there is to manually install grub, from the live-session.
In order to do that you have to tell the « live-session » to target the / root partition of the system installed on hard disk in the computer ( and not its current actual root which is the one from installation media )
Hence the chroot command ( change root ) and the others mount command that tie together the needed folders on hard disk.
Once « chrooted » what you install from the « live-session » actually ends on the hard-disk of the installed system.
[ hope my english is clear enough - and my explanation accurate - welcome anyone for correcting it ]
Anyway you are right : all that should not be needed. Let’s hope it will get fixed in due time.