Many free things can be a scam if you are promised to do something and it laters fetch you no reward. For the fact that you wasted your time, data, energy in doing the free stuff and no rewards come after, then you should know that it's nothing but a scam. They might have used you to achieve their purpose which is joining their Telegram. They needed people to join their Telegram group which they might want to use for another purpose, the only way they can get it is to announce a fake airdrop, then you start joining and when it reaches the number they need they tell you the form is closed and few months later they divert the group to another thing. Tell me how that is not a scam.