Most groups on telegram are actually legit and there are some that came from popular sites but fake groups or unoficial groups arent really mean to scam inoscent people . they are built to discuss the said site and also the blockchain giveway that you are talking about is not a scam but its 100% legit if you will only login on your wallet , you will see that they are airdropping stellar lumens .
I think he wanted to say that a few weeks ago a fake group was imitating Blockchain XLM airdrop program, which is why we should be more careful because it's fake and it aims to steal your money.
Anyway, long story short, I nearly fell to this kind of scam when I wanted to join an ICO. The fake channel uses the exact name of the ICO that I wanted to join, and oddly enough there is no mention about it on the official group even though the members on the fake channel reach thousands of people. I wonder if there are more cases like this.