No, it's free. They have a whitepaper on their website: https://getsession.org/whitepaper

All in all there's a pool of tokens that gets paid out to the stakers. The full network of nodes determines what nodes are eligible by testing each other. The pool gets a constant flow of tokens over time, while transaction fees and specific purchases (like a custom username instead of one of those long IDs) feed the pool as well.

Keep in mind I'm not here to sell it. I really wish it was more like free Tor nodes, in which case I would be hosting one already.