From what I read in their documentation[1], session's network is built upon an onion routing system where nodes are registered in the OXEN blockchain; you stake 15000 $OXEN to create a node and if you are "well-behaved" you get rewards in that blockchain -- otherwise you lose your stake.

I haven't dug too deep but it doesn't sound much more centralized than the tor network for example.

[1] https://docs.oxen.io/products-built-on-oxen/session/network-...