This is kind of interesting. I grabbed the 3.11 build from your link, @Megalomaniak. On trying to open the instancing file, got this dialog:

I chose "OK". After loading I got a series of errors in the console. Then I discovered a missing feature (and a pretty easy and desirable one) in Godot: you can't copy text out of the Output console.
There are lots of errors of the form
PNG Corrupted
and
Error importing: res://instancing/__MACOSX/instancing/assets./Bouncy Platform Medium.png.
That last seemed to indicate that Godot is not acknowledging the current directory for some reason. I have the instancing files stored in a Dropbox folder. On Linux, the Dropbox client mirrors your file tree from the cloud service to a local ~/Dropbox folder. I had unzipped instancing.zip two levels of directory below that. I cd'd to ~/Dropbox/Games-ESC/Godot-Projects/instancing and ran
~/Downloads/Godot_v3.1.1-stable_x11.64 ./project.godot &
And that works fine. The problem is apparently that the way the Godot engine interprets "res://path" is not based on the path to the .godot file that you just opened, so it can't find any of the resources referred to. "Apparently," I am not saying I fully understand.
Anyway, I know the workaround now. Thanks.