It is true that many (although not all) medieval thinkers treated Aristotle as something like a sacred text, perhaps because Thomas Aquinas had used so much Aristotle in his theology. But we can't blame Aristotle himself for that, anymore than we can blame Jesus for the Spanish Inquisition. It is also true that Aristotle did say "We see that bodies which have a greater impulse either of weight or of lightness, if they are alike in other respects, move faster over an equal space, and in the ratio which their magnitudes bear to each other."(Aristotle physics book 4 part 8). Note, however, that he says "we see this". In other words, he based this claim on an observation, and given the technology of his time, it was a reasonably sound observation. We can all see that a feather falls more slowly than a piece of cloth, and a piece of cloth falls more slowly than a rock. Without more precise measuring tools (you can't measure the speed of a falling object with a sundial), Aristotle's claim seemed observably true.

Aristotle actually did believe that a feather and a rock would fall at the same speed in a vacuum. However, he used this conditional fact as proof that a vacuum does not exist, because we can obviously see that heavy and light objects never fall at the same speed. In other words, he used this observation as the basis for the following modus tollens argument:

If vacuum existed, then heavy and light objects would fall at the same speed.

Heavy and light objects do not fall at the same speed.

Therefore vacuum does not exist.

It is not surprising that he was unaware of the fact that vacuum exists in deep space. What is surprising is that, contrary to popular belief, Aristotle realized that the only thing that stops objects of different weights from falling at the same speed is the resistance of the medium through which they fall. See the original context of the Aristotle quote cited above for more detail.