The airdrop takes only about 2 minutes and the return of $20 is insanely good. However it's not clear how many DNN tokens you receive. Due to the current ETH price it'll be around 100 DNN if you do all tasks.
All in all a very good project with an even better airdrop!
Quote from the admin: "The presale was actually slated to end on April 3rd, which is when the public sale would have began. But due to the current market climate making it a bit difficult for token sales right now, we've decided to postpone the public sale until the market improves. The moment we have a fixed date in mind, we'll announce it on all our community channels :)"
The airdrop was announced on their official Telegram from admins. Therefore it should be legit.