Do they ever take accountability? Sometimes. It depends on their self-awareness and how high-functioning they are. When he's not dysregulating, my H has a lot of awareness and he'll admit to and apologize for a lot. But that's after a breakthrough in anger management therapy -- and he was already high-functioning to begin with.
But that depends on the case.
My wife acts like a child and threatens to divorce me at least once a day.
What a coincidence! My H told me he wanted a divorce last night (he, of course, does not).
But seriously, it's a difficult thing to hear, though I've found that it loses its sting over time. I've warned him now that he needs to drop that threat unless he really, truly means it. It could have very negative consequences for him. But, as I said, he's self-aware when he's centered.
Is your wife still doing therapy (outside marriage counseling)? How does your marriage counselor handle it when she talks that way in-session?