The idea is that most vegans online lie about being vegan (or people will claim to be vegan and secretly eat meat) and therefore:
If someone claims to be vegan and isn't vegan, then they aren't a vegan, simple as that. It's weird to say that this discredits the position that they claim they want to hold. Can I just say "I'm a Muslim, but I don't believe in Allah" and then would I be able to turn around and say "See, Muslims don't even believe in the God they claim!" This is not really a good argument against the position
Plenty of murderers pretend to be non-murderers (until they get caught). Can I now claim that "non-murderers are hypocrites because they actually secretly kill people"?
Whatever vegans do or don't do has nothing to do with the ethics of killing an animal. If no other vegans existed on the planet, would it somehow make it moral
People get upset when vegans are overly policing and judgmental of reduction-limited ethical systems or part-time vegans. So which is it? Are vegans not policing the term vegan hard enough or are we too soft and need to police it harder? Seems like it's just more attempts to filibuster with bad arguments or throw everything at the wall just to see what sticks.
Also, wait, was veganism not rigidly defined enough? So then how can you tie what people report to what they actually are? Surely if veganism is just whatever the person says it is, then 100% of claimed vegans are actual vegans. Or if there isn't a good enough definition, then how could you even establish such a metric to begin with?
If someone "more or less eats just plant-based" then are they vegan? And if they eat animal products once a week are they quitting veganism 52 times a year? This whole argument falls apart upon inspection.