I have never heard "you're denying them agency" used in a way that deepened analysis.
Imagine, you're sitting around with friends attempting to describe a system of oppression. You're sussing out the forces that impact the lives of the oppressed. Suddenly, someone who's had very little to say busts into the conversation to let everyone know that they're "denying the agency" of the oppressed.
Are they suggesting that oppression is irrelevant to those oppressed by it? No, that would be silly. Are they saying that your analysis views the oppressed as fully determined by their oppression? They might be, and you should consider whether it does, but it probably doesn't, because you're not a shithead, and it's actually kind of hard to accidentally say that the free will of the oppressed is muted entirely by the oppressor (and because you've read Domination and the Art of Resistance by James Scott, and it was dank).
Are they engaging in some vacuous moral posturing? Probably.