No, it's not. And there's no such thing. Nothing is completely decentralized, except grey goo vel cetera. An internet, fundamentally, is a noncentralizate. Nonetheless, a centralizate can comprise noncentralizates.
I mean I guess the Internet technically is not completely decentralized because of ICANN, but I think that's an abstraction layer below what we're talking about here. Decentralized is defined as not centralized. So at the level of abstraction we're talking about the Internet is decentralized but not distributed, there are lots of servers, which aren't centrally planned, but there is still a client server distinction.