If the post-2008 world isn't proof enough that printing money does not automatically increase aggregate demand, I don't know what would count as proof in Econ.
By the way, I'm not saying it can increase AD, just that it does not automatically does so, for some reason that is widespread now.
The central bank can always inflate. If printing lots of money isn't enough, they just need to print some more money.
And, if they manage to buy all of government debt without creating any inflation, that's awesome on some level as well: they just managed to monetize all the public debt without any adverse side effects. (And for sovereign debt, monetization is about as good as eliminating it.)
By the way, I'm not saying it can increase AD, just that it does not automatically does so, for some reason that is widespread now.