Airdrop or free tokens has been exist for a long time ago, I think you already know there's so much airdrop on bounty program and still exist till now.
Honestly 400 token distributed from total supply 1.000.000.000 is quite small, but since the rates of each token is 4-7$ it makes you've got a jackpot (1600$ - 2800$).
they just got lucky for this UNI token. but OP should know that luck doesnt come everyday. though there are so many airdrops that you will encounter, hard to tell which one will be your lucky one. but one thing you can do, is unearth their background. if the foundation is good and solid, you have the chance that your airdrop will make good money once it hit the exchanges.
but remember, almost all these airdrops are worthless. sometimes you cant even pay the gas fee/withdrawal fee in exchange of the free coins that you got. so i dont think someone can give you the list of the next UNI token...