I don't agree with this. I suffered from hyperopia my entire life, with a +5 prescription in both eyes. Total blurriness without glasses, both near and far. After 2-3 years of constant eye exercises and gradually lowering my glasses prescription with custom lenses I purchased online, I am now able to see very well without any glasses at all. I still use a +1 to read, but I believe with additional training I can train the fine focusing capabilities of my eye.
I know tons of people who have had eyesight improvement when they either stopped wearing glasses or wore weaker pairs. There is also entire communities of people online who have had amazing success with training their eyesight - it just takes longer than training muscles. Like, daily effort over the course of 1+ year.
Now, I don't know if the ciliary muscle is actually getting stronger. It's very possible my eye is just becoming used to putting more tension on the ciliary muscle. The ciliary muscle can definitely be flexed when commanded by the nervous system, as that is the primary mechanism of how we focus.
Likewise, it is also known that when continuous tension on the lens/cornea of the eye reshapes it temporarily. We know this from Orthokeratology.
In the case of Hyperopia, it is thought that the ciliary muscles are too weak. In the case of Myopia, it is thought that the ciliary muscles are too strong. In the latter case, it is more about lessening the activation of the eye muscle, which can be conditioned by training exercises specific to relaxing it and refocusing it.
Nevertheless, all of what I said is real and there is tons of evidence for it online. Tens of thousands of people have improved their eyesight through the above theory. Doing so is certainly possible. But like any muscle in the body, the eyes need rest when they're being stimulated like this. It's gradual and not like someone can just drop their glasses and try to see without them from then on - unless their vision was already close to good.