So I think to say that he simply affirmed that aggregate demand exists is underselling his contribution - he managed to convince all the important policymakers of the time that it was important enough to overturn their prior thinking about how to use fiscal (not monetary) policy in a countercyclical manner.
The scare quotes around money printing don't help progress the dialogue around the mechanics and consequences of government spending either. Today with a fiat currency system, the government can enact countercyclical fiscal policy by simply crediting private sector accounts, and increasing a balancing government liability electronically (note there are _no taxes involved in doing this_). No money is 'printed' (the term conveys unrealistic connotations), private sector demand deposits are simply increased electronically.
The increase of these demand deposits is not inflationary unless they are used to purchase goods and services in excess of what can be supplied at a constant price level. So when we find ourselves in a situation where 20% of the workforce is suddenly unemployed due to a shutdown of economic output, the government can spend to help stabilise aggregate incomes without risking inflation, because no-one is buying stuff otherwise (and as a result, no-one would be making any income).
It is certainly true that a number of economic schools of thought have tried to use his name to legitimise their ideas, and he'd likely be disappointed with a few of them.