While the 3GPP Profile and Spec aren't exactly "Open" by Open Source and HN's Standard, It is definitely not closed or proprietary. You can implement a working 3G / 4G / 5G system. It just wouldn't be competitive with commercial offering.
And US actually has plenty of IP via Qualcomm in Core Standards. Ignoring IoT and AV ( Autonomous Vehicle ). Also partly via Nokia when acquired Alcatel-Lucent, where Lucent is from AT&T Bell Labs.
So No, US aren't out of the 5G circle. They just dont have any companies making specific part of the 5G infrastructure.