First of all - your guesses are are very good and almost accurate.
Second - 1669 is currently on gradual deployment so the SK92449 will be updated with exact HFoHF documentation when it gets to 100% GA (will take around a week).
Now to the point: The behaviour in the last years was that when a new HF build was installed, the old one was uninstalled as a first step (automatically by DA).
However, this caused issues to customers who got anothe HF in the middle which was included in the new build. The DA could not uninstall the old build since something was installed on top of it.
The new behaviour: The new take will be installed on top of the old one even if there us an HF in the middle as long as the HF in the middle is included in the new build.
Another advantage: Now you can uninstall last take and get back to previous one (since we didn't really uninstalled it) while untill now you had to uninstall the new take and then install manually the old one.
Hope it clears things and I'll be glad to get feedbacks by email.
Thanks