I think looking at it as a continuum makes some amount of sense, though in this case the authors of the paper seem to have done lots of due diligence in terms of doing the chemistry to check known production paths.

As in, there is known chemistry that produces phosphine at the temperatures and pressures that exist on Saturn and Jupiter, but there is not known chemistry that produces phosphine at the temperatures and pressures that exist on Venus.

So for this particular chemistry, the difference between "gas giant" and "rocky planet" is still a helpful one.