I solved it!
I hadn't created a C# solution because I wanted to play around with the addon in a new project.
Seemingly, C# addons can't be loaded in Godot .NET without creating a single script and building the project.
Sorry for the confusion!
I tried to load the .NET version of the addon in Godot 4.7.2 .NET on linux but got a generic error message:
Unable to load addon script from path: 'res://addons/mold/Editor/MoldPlugin.cs'. This might be due to a code error in that script.
Disabling the addon at 'res://addons/mold/plugin.cfg' to prevent further errors.