Insurance denied claim for hospital stay but says I shouldn’t have to pay for it anyways?

Back story: I went to the emergency room for something and got admitted to the hospital where I had a 3 day stay. The insurance accepted all the claims sent from the hospital except the hospital stay (the most expensive). I have appealed the denied claim once and just got their response back. While I believe I have grounds for the claim to be accepted, the appeal denial letter had an interesting paragraph that could make it all irrelevant but I am not sure. Please help me know if it can be substantiated:

“Kaiser Foundation Hospital is in your plan, so you shouldn’t have to pay for the services in question. You’ll only have to pay if you are told that your health plan won’t cover them and you sign a service-specific agreement before getting the services.”

To my knowledge my spouse never signed a service specific agreement, and I was unable to sign anything. So is it true that we do not have to pay? or will the hospital just try to charge us anyways? Is there any law or something concrete to substantiate such a claim? Should I appeal the claim again or is it a moo point?

I live in Oregon and have Anthem BCBS if that matters. Let me know if you need more info. Thank you in advance!