Or in other words, you should derive the disk’s encryption key from the user’s login password?
This also allows people changing their password as you do not change the actual (strong) key used for the disk but the key used to access it.
No, it's not. I use a book to generate a string of numbers and I can write them on a sheet of paper. If I put that paper in a room and lock the room with a key, I need the key to access the paper, but the numbers on the paper are in no way derived from the door key.
You are incorrect in your understanding of the word derived.