Actually, scale-to-fit looks like to be a simple shell script, part of phoc:
https://source.puri.sm/Librem5/phoc/-/blob/master/helpers/scale-to-fit
Doing a search on the forum, I found this info how to download and use it: https://forums.puri.sm/t/librem-5-nextcloud/10434/7
But that might be too much. If you just want to do a simple test, you can just write this command (the main thing that script does):
First, run the application, cherrytree, as you normally do [EDIT2: or maybe the command need to be run first, IDK]. Then, in the terminal do:
gsettings set sm.puri.phoc.application:/sm/puri/phoc/application/cherrytree/ scale-to-fit true
In the above command, you can replace cherrytree with the name of another application you want to scale, making sure you use only lowercase letters and digits (if any).
Tip: in the terminal, if you use the Up Arrow, you can reuse your last commands entered.
Edit: I haven’t test all of these, I don’t have yet the phone. I saw your video and was curious how would that scale. I assume it won’t be too usable after scaling since the widgets/texts will be too small.