It boils down to the BE-4 engine which Tony Bruno at ULA is betting the farm on.
In the meantime, ULA continues to use the Russian RD 180 engines as usual.
Bezos knows they won't sue when he can't deliverr, at least not right away.
HOW ROCKETS ARE MADE (Rocket Factory Tour - United Launch Alliance)
https://www.youtube.com/watch?v=o0fG_lnVhHw
Tory Bruno talks about Rocket Engines and ULA’s Business philosophy
https://www.youtube.com/watch?v=DQaPOIQLEUo