I think they're implying that if you want to test to effects of microgravity, you don't want something like 14 psi of air pressure happening at the same time, b

I think they're implying that if you want to test to effects of microgravity, you don't want something like 14 psi of air pressure happening at the same time, because interactions with the air flows might confound whatever you're trying to test. The station is likely better in terms of gravity, but in terms of test conditions, you're somewhat limited by the astronauts being there.