But that is certainly not what is being claimed here. What TFA says is that they think Leonardo correctly realised that falling things accelerate, as opposed to fall with a constant speed as (the ancient) Aristotelian physics prescribed.

To go all the way to the (Einsteinian) equivalence principle from that is huffing great man hopium in the extreme: Leonardo is certainly not as user of reference frames, and the equivalence principle simply does not exist without them. Another problem is that he didn't know of inertia, what with being of the Aristotelian school.