All contributors to the OpenJDK project must agree to the Oracle Contributor Agreement (OCA). The OpenJDK contributor page summarizes:
The OCA gives Oracle and the Contributor joint copyright interests in the code. The Contributor retains copyrights while also granting those rights to Oracle as the Community's sponsor.
In other words, Oracle is the copyright holder (or full rights co-holder) of all OpenJDK code, including submissions from outside contributors. Oracle's right to include OpenJDK code in their proprietary JDK stems from their status as copyright holder, not from a GPL license grant. Therefore, they are not required to follow any GPL requirements in their distribution or reuse of the code.