A scientist has the subjective experience (qualia) of observing a person sleeping with certain scientific equipment attached to them, the subjective experience of observing that equipment produce certain results, the subjective experience of waking the sleeper and asking them if they were just dreaming and getting an affirmative response, etc. If the scientist claims that "dreams are brain activity", their claim is referring to those subjective experiences of theirs, and has those subjective experiences as its justification. And that's all fine – there is no problem with any of this from an idealist viewpoint. "Brain activity" is a pattern in qualia, "dreaming" is another pattern in qualia, some correlation between them is a third (higher-level) pattern in qualia. It's qualia all the way down.
But, to then use those subjective experiences as an argument against the existence of subjective experiences is profoundly mistaken.