This somehow invalidates their claim because they need to clean up their health before they can instruct others
Carnists will frequently claim that someone out there [theoretically has a medical condition that is helped by carnism. But does this excuse not also apply then to this condition? This person X probably looks sickly due to some unspecified medical condition.
You can't be too skinny to advocate for veganism, you need to be in shape (even though carnists will claim that a carnivore diet helped them lose weight). But too good of shape and carnists will claim that you're on the juice, or secretly eating meat, or gained too much physique before veganism. Or they can just claim you're only in shape because you're on steroids.
So you would need to be in the middle, but of course, then one can sit down with dozens of pictures of you and other youtubers with reported bicep measurements and in the end, come to the completely objective conclusion that veganism can't build muscle. Also, if you haven't been vegan for long enough, your physique is still attributable to carnism. I'm sure there will be a settled number with no goalpost moving.
Also, assuming we identify the perfect level of in-shape to be in, that doesn't mean you know anything about nutrition; it just means you're at best a stopped clock that is right twice a day. So you need credentials. You might need an MD or you might need to be a registered dietitian or nutritionist. Whichever you are, I'll claim it's the other one you need to be.
In conclusion, you really need to be someone that's in shape, but not too in shape - it's very specific, must be a nutritionist/dietitian/medical doctor, that's been vegan for "long enough", and must come with documented proof that you had no genetic advantage of any type, and pictures must be submitted of you in the act of eating every meal for the last 10 years to prove you've been vegan, as well as pictures at the time when you claimed they started veganism to prove that it wasn't just carry-over fitness from carnism.
That's the only person I can take nutrition advice from.
The conclusion of this argument is tied to the perceived least-healthy-looking person ever recommending a diet. But that obviously has the problem that it doesn't tell you anything about the diet itself, and just because one random weirdo decided to eat nothing but cantaloupe for 10 years and is dying of nutrient deficiency doesn't mean that you are going to suffer the same fate if you eat a balanced plant-based diet.