Why that end works

Because it acts on both systems at once, and it is the only ordinary habit that does.

A fixed rising time holds the body clock in place. The clock is set mainly by light and by regularity, and a wake time that moves by three hours on a Saturday moves the whole system with it. That is why Sunday night is so often the worst night of the week.

And it protects sleep drive. If you get up at the same time regardless, the hours you were awake do their proper work and the pressure to sleep builds where it should. Lie in to repair a bad night and you spend that pressure in the morning, so there is less of it left for the evening.

Notice that this is not sleep restriction. You are not being asked to cut your time in bed to a target. The bedtime end is left alone, and it tends to sort itself out over a couple of weeks.