Old Scam school. He tries to scam others to join his group and then sell real followers.
No one will believe that you will get free money to do an easy task, but since it is an easy task, many feel greedy, especially after reading one of the positive responses.
He could gain some credibility if the cost of joining is 10 Satoshis and payment by lightning network.
It's a bit weird, I don't think so, usually, these scams are like, send us 0.01 BTC to verify you are a real person, and we will send you 1 BTC of a coin that is pretty well traded. I've also seen them try and build big telegram channels, some project buys it and rebrands it and suddenly they have a community of 10,000 people.
There are 2 possible explanations for this in my opinion.
1. Just someone who likes to fuck around with people and waste people's time (most likely)
2. Possible member of the GRAFT team trying to get more members in the chat, with a fake telegram account spamming their fake airdrop.