That is nonsense. I know that there as posers who claim to be vegan but actually aren't. They might eat oysters. People who actually are vegan don't eat anything in the animal kingdom or their products.
Oysters are in the animal kingdom, therefore vegans do not eat them.veg·an
/ˈvēɡən/
noun
noun: vegan; plural noun: vegans
a person who does not eat any food derived from animals and who typically does not use other animal products.
It is not necessary to evaluate the sentience of each creature. If that was the standard, I can think of some people it would be okay to eat. To be vegan, the standard is whether or not it is in the animal kingdom.
Om mani padme hum
Kathy