That is a good point.
We can go back to Adam and Eve’s time, and point out any part of the world and say that it wasn’t part of that country or this nation.
Indians talk from both sides when they say accept line of control as “border”, then use pathetic excuses like “lack of boundary marker” to send in their troops.
However the point is not to decide whose territory is Siachin at this stage.
It is just that Pak and India are in the state of War in Siachin. And any trade relations will greatly benefit if they agree to give up military ambitions along the border.
Got it?