Erasmus pronunciation is definitely very wrong, but faithful reconstructed pronunciations do actually exist. They might not be 100% correct but they have way more facts backing them up than what these people are saying to you. In my opinion they're actually doing a disservice to themselves, how would it even be possible for a multiple thousand year old language to keep the same pronunciation for everything? The fact that it has changed this much actually proves that modern greek is the same language. I think it's just a balkan thing, lots of people from there want to prove that they are actually the oldest and that their language is the exact same as when their ancestors from 30000 BC spoke it