I noticed you forgot to include a very important contextual sentence for your myopathy quote:

Only three studies [27,29,30] have carried out hematological and/or biochemical analysis of blood in cats that were fed vegetarian diets, and it is worth noting that sample sizes were low. Cats on a high-protein vegetarian diet exhibited hypokalemia which accompanied recurrent polymyopathy [29]. There was also increased creatinine kinase activity, likely reflecting the muscle damage caused by the myopathy, and reduced urinary potassium concentrations. Potassium supplementation prevented development of this myopathy, strongly suggesting a link between the potassium and myopathy.

Meaning there was a health problem when one of the cats' dietary needs wasn't being met, which no longer appeared when the deficiency was corrected.

Even so, no one was trying to claim every conceivable vegan food mix is healthy for a cat. Of course trying to switch an animal who would be a carnivore in nature to a healthy synthetic vegan diet would be difficult. But there only needs to be one diet that succeeds to show it's possible. And unless you're going to claim literally all of the vegan cat guardians who reported healthy cats are lying about their cat's health or diet, that requirement has been met.