Is it really developed on Windows if Windows is solely only used as thin client to access something else? I'd say for examples #2 and #3 the development OS is definitely Linux.
#1 is the only where I'd lean towards "Windows", since WSL tends to involve a much more split workflow, where a lot of your interactions happen outside the VM.
Why is #2 not Windows? He is in Windows. That remote machine hosting a notebook may tomorrow be replaced with Windows, and he would barely notice if he didn't put a lot of shell in his Python.
#1 is the only where I'd lean towards "Windows", since WSL tends to involve a much more split workflow, where a lot of your interactions happen outside the VM.