Having a .NET 9.0 test project depend on a library that depends on Polyfill doesn’t compile

I have a class library depending on https://github.com/SimonCropp/Polyfill

Having a .NET 9.0 test project depend on a library that depends on Polyfill doesn’t compile

I have a class library depending on https://github.com/SimonCropp/Polyfill