I don't think that's the right interpretation - albeit it's a reasonable one that I've made before. I believe it is interdicting adding in additional international segments if the remaining itinerary is purely domestic.

That said, I've not seen such historical pricing before departure provisions before and wouldn't have thought they make much sense. There are legitimate reasons to open lower bucket space close to departure but not want to allow people like OP to just cash back out on something they already bought having locked in a much earlier advance purchase date. It may also be that the real problem is hiding elsewhere in the rules. Many agents aren't all that good at parsing and interpreting them but merely rely on what the computer says when given the new itinerary, making them unable to provide the reasoning behind a reprice regardless of whether it is correct.