Ahh nuts, you are correct. This functionality has changed with 2.9 with the introduction of Face Sets. Instead you can make your edit-mode selection into a face set and tell Sculpt Mode to only modify said face set:
I 100% approve of this idea. If only to smoothly finish the course and translate the understanding later to the new version. It’s a good learning option 👍
Just FYI in case anybody comes across this and is confused, the previous functionality has returned – tested in 2.91 and 2.92 and face hiding between Edit & Sculpt modes once again works just like in the video.