Because like a gas giant, the surface is effectively unreachable or at least absolutely uninhabitable (if it even exists for a gas giant). The atmosphere of Venus, like a gas giant, becomes a very hot, supercritical gas as you go deeper. Like a gas giant, the only potential habitable area is in the atmosphere.
Venus still has a sensible surface, although it's unclear if that concept makes sense for a gas giant. "Gas dwarfs" exist, which have a thick atmosphere around a rocky planet (i.e. around Earth's mass), but Venus probably doesn't qualify for that. It's sort of a continuum, though, so the dividing line is somewhat arbitrary.
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.
In an colonization sense perhaps, but the environment is very different. Pressures while high on Venus are orders of magnitudes lower than in gas giant atmospheres.
Venus still has a sensible surface, although it's unclear if that concept makes sense for a gas giant. "Gas dwarfs" exist, which have a thick atmosphere around a rocky planet (i.e. around Earth's mass), but Venus probably doesn't qualify for that. It's sort of a continuum, though, so the dividing line is somewhat arbitrary.