In general, this is definitely something to watch out for. As far as I understand it, that's not the case here, however. These coins were already in existence

In general, this is definitely something to watch out for. As far as I understand it, that's not the case here, however. These coins were already in existence held by Stellar, who is now giving them away. I.e. no coins being printed. I see airdrops regularly, and this is the first one I have not dismissed as a waste of time and attention right away. Unless they mess it up somehow, I think this is a great move by Stellar - I think the people using Keybase (especially combined with GH/HN) are exactly the early-adopters-power-users they want to extend adoption to.