That is the logical explanation for sure, and the one that is the lesser character assassination, but it's also one that requires a bit of obscure knowledge on Arthas and Sylvanas' relationship, as well as being the option that doesnt make excuses for Sylvanas' actions.
In pretty much every way it's the better choice narratively, but the writing around Sylvanas is hardly the kind that deserves the benefit of the doubt at this point.