OK, just tried out the unencrypted install and it works as expected. 
I assume you didn’t choose to run #07 from the guide.
By default btrfs-grub only seems to update grub (by hook) after a system update. It doesn’t run if you invoked the creation of a snapshot manually or if Timeshift creates them by schedule.
That’s why I wrote a systemd service (#07) that deletes old snapshots and runs grub-mkconfig on shutdown.
It’ll slow down shutdown by a few seconds but I can live with that.
So, you could try creating that service and reboot (a couple of times) and your snapshots should show up in the boot menu eventually.
[Edit] Regarding my former post: Timeshift was set up correctly; no need to do this manually from inside Timeshift.
[Edit 2] I’ve been meaning to change that shutdown service to a path unit that monitors the snapshot folder for changes but haven’t gotten to it.