In all fairness, I have not read this yet. But my initial thought process when I see "a protocol for distributed transactions across microservices" involves a dose of skepticism. If you need distributed transactions across microservices, why not just provide an XA compliant interface, and then plug in to an existing transaction manager? And, probably more to the point, while EBay specifically might need "real" distributed transactions, I suspect that most organizations rarely - if ever - need distributed transactions across microservices, and should instead use the Saga Pattern[1].
[1]: https://microservices.io/patterns/data/saga.html