Ancient philosophy would look very different if the Greek philosophers had been making stuff up at random. Plato and Aristotle followed cognitive strategies, strategies that they (1) could communicate and (2) felt constrained to follow. For these reasons, I don't think that those philosophers could be characterized in general as "making stuff up".

Of course, they followed different strategies respectively, and they often couldn't communicate their feelings of constraint to one another. And of course their strategies often just didn't work.