Hiya!

In short, because Dan Carlin is wrong. He comes up fairly frequently in questions and assumptions here, and you'll find that most of the /r/askhistorians community, while acknowledging his important role in stoking an interest in history amongst the general public, also cautions against taking him at face value. Statements like this are an excellent example of the vague and spurious claims Carlin is prone to making - there's obviously no sense to such a statement, and countless examples of long, devastating conflicts exist to disprove such a statement, from the Peloponnesian Wars to the Napoleonic.

Without having the context for Carlin's statement here, I assume that Carlin is referring to changing strategic trends in Europe as a result of industrialisation and increased manpower. While these changes certainly altered the face of war in the 20th century, this alteration should definitely not be characterized as states 'being able to take a punch' for the first time. Situations where states survived 'punches' - decisive military defeats - to continue waging an extended, devastating conflict exist in abundance in European history alone from antiquity to the 19th century, to say nothing of the rest of the world. Carlin is attempting to make a valid point - the nature of conventional conflict was changing - in an overly simplistic and misleading way.