The idea here is that no one could actually be vegan, so vegans must be faking it
Is it not a guarantee of health to be vegan because you can still adhere to a diet of beer, skittles, and junk food... or is it so hard it's impossible? These can't both be true even though carnists will argue both.
Also what about unhealthy self-identified vegans, these people are not actually vegan so then how is it that they are sickly? So these people aren't vegan but they are suffering the consequences of a diet they aren't actually following.
It's impossible to invalidate this argument because it requires following everyone around at all times with laser-precision proof. So this is just a feeble attempt to throw an unprovable fact out there.