Only a half truth. Borders may have been loosely defined but they were absolutely defended with violence.

Yes, but the means by which that state violence was organized and carried out often looked very different. Obviously there was some sort of distinction between medieval lordships or what have you, but the organizational form of the modern nation state wasn't codified until the Treaty of Westphalia, which ended the 30 years war. This was co-constitutive with the enclosure of common land, and the birth of modern capitalist property relations.

But the nitty gritty details are besides the point. The main thing I'm stating in my comment is that OP is making a falacious appeal to nature. As though a dog pissing on a rock somewhere says anything at all about how humans should conduct border policy.