It's using a distributed hash table, not trackers. Nothing's completely decentralized; distributed hash tables are pretty good, since you can join them by talking to any participating node at all.
I don't know what would be the term for it since it is not completely centralised, as in the nodes collectively form the system without any central coordination, but a new node joining in still needs some information to connect to the network from some source which could be denied-of-service by say a government, so not completely decentralised either.