The point here is that for Dennis Ross to say “Jerusalem is Israel’s capital, and Jerusalem must never again be divided” is not the same as saying “Jerusalem cannot be the capital of an independent Palestine.” The US envisions a future in which Israel considers Jerusalem its capital and has sovereignty over Jewish neighborhoods, Palestine considers Jerusalem its capital and has sovereignty over Arab neighborhoods, there are no hard barrier walls dividing the city, and security and other municipal arrangements are worked out in negotiations.
So undivided means a city divided under two separate sovereignties but without any actual wall or barrier to police it. And that's a fact. Yes, there's a reason this stuff is esoteric. Plain English cannot account for the political and rhetorical bullshit on all sides.