Well, I'm dumb. What happened: I'm using VS, which also comes with a vcpkg installation (that I had enabled, for testing). And I believe that this installation was used to fetch packages from github, but was an older installation and did not allow packages from the specified git commit. And it returned the misleading "<package> does not exist".

So the solution for me was to point the VCPKG_ROOT enviroment variable to the proper vcpkg installation directory (you can do this by setting "environment" inside CMakePresets) ... and now it works. Thanks everyone for trying to help!