Insurance denied coverage for hospital stay, but says I won’t have to pay?

I have health insurance through my employer which covers 100% of an ER and subsequent hospital stay if you are admitted ($150 copay if not admitted). I ended up in the ER a few weeks ago due to severe pain and ended up having emergency surgery. The surgery took place at 4am and I was admitted so they could “keep an eye on me” until I was discharged at 5pm the same day. I received a letter today addressed to the hospital stating that because there were no complications, such as excessive bleeding, my insurance company did not think I needed to be admitted and is therefore denying coverage. However, the letter also stated that patient will not have to pay due to coverage denial. But…how? Won’t the hospital bill me if insurance won’t pay? I am calling my insurance company first thing tomorrow as they are currently closed for the holiday, but can someone shed some light into what this means? Also any tips on how to handle the call tomorrow would be greatly appreciated!