It kind of seems to me that "the modern territorial state sprung into existence with the signing of the Peace of Westphalia" is something of an axiom within academic polsci. However, I often see it being mentioned as a throwaway statement by legal and political science scholars that don't necessarily have a history background, and it rarely seems to be engaged with in any critical manner. Was the Treaty itself really that pivotal of an event? How can it be reconciled with notions of statehood held in non-European contexts?
For your interest, here are some answers to similar questions.
u/mikedash has answered Why is the Treaty of Westphalia significant?
u/sunagainstgold has answered What ideas did ‘Westphalian sovereignty replace?
u/Aeoleth has answered If the Peace of Westphalia established a new way of doing business between European states, what was it replacing?
and a since deleted user some time ago now answered What was the international system (for lack of a better word) before the Peace of Westphalia in 1648?