There is nothing stopping Bittorrent clients from agreeing on "this dht will be global", and similary, what stops IPFS clients/peers from forming a different DHT? Nothing as far as I know, not like scuttlebutt which has a transport-layer network-wide key.
What about trackers, bittorrent can but does not work well without trackers, so its not really according to me, decentralized, since some peer is more equal than another peer.
IPFS is meh, datproject is where its at.