This seems like a vague sense of displaced blame, however; so I can understand how people will view it as a red flag: There's nothing preventing her from leaving of her own volition but it's inferred that she has no possibility of choice (without him having to have made a choice) - even though he's already signaled his desires multiple times before.
In that sense, it is an ultimatum but one that tries to absolve guilt by making the "decision-maker" the "owner" of the result/consequence - even though, in reality, both would suffer the consequences of the choice.
Further still, is the impression inferred that she's a prisoner because she's "bound" to him because he needs to "let her go" if he won't fulfill her desire[s]; inferring that, somehow, he - alone - wields power over her self-agency.
That doesn't quite make sense - when a healthy relationship should be two independent people sharing in the relationship "power" dynamic. In other words, it should not be so skewed that one has power/control over life, destiny, etc. of another.
In this perspective, it does seem to be a very troubling red flag.