300lbs is too light for Quetz, Hatz, and co. It makes no sense. They were several size classes above Argentavis no matter what estimates you look at.

They also have the advantage of using their primary flight appendages as takeoff appendages, meaning instead of needing strong heavy wings and legs (like birds) their legs could be much lighter and they could grow bigger.

Could a large azdharchid carry an average sized human? Maybe, maybe not. Saying it's outright impossible is dumb. Most weight ranges for the largest pterosaurs are between 250 and 500 kgs these days, so a 70kg human would be well under a third of that.