I can’t really say whether it’s ableist or not. Nonetheless, I do have some thoughts on the matter.
I’m rather certain that, for all of Hawking’s career, he was a staunch materialist, i.e. he was no Cartesian dualist—he was committed to the idea that only the material world exists (though discussion of simulated and parallel universes can easily be found within his body of work).
This means Hawking himself would never consider it possible to be “imprisoned by” (and therefore “freed from”) his body, because he is his body. A prison cannot be imprisoned within itself.
Indeed, while some parts of his body made many aspects of living difficult, other parts (namely, his brain), opened up whole worlds for him to explore and share
So, I believe, Hawking would argue that saying he was “imprisoned by his body” is just nonsensical.
Does that help inform whether the statement is ableist? I’m not sure.