While not very well described in the whitepaper, the system actually uses token commitments to verify source nodes. Relay and exit nodes verify that the connecting source node has enough token committed (locked up in an Ethereum smart contract) before and after accepting connections / payments.
The only two operational systems I'm aware of using probabilistic payments is blockchain Proof-of-Work, where mining rewards can be viewed as probabilistic (though with annoyingly high variance) and blockchain-based lotteries.
IMO probabilistic payments can replace payment channels in quite a few blockchain-based systems such as https://gridplus.io/ (basically in systems/apps where a service is provided that is (very) granular and continuous (video streaming would be another example))
This is the first I'm hearing of probabilistic payments. Are you aware of any other systems, proposed or operational, using this to pay for bandwidth? It seems like a natural fit.