A new Reddit post shows a messaging thread that may have taken place between a OnePlus representative and a second party. The former mentions the date now linked to the official release of Android 10, and references the OEM's plans to release its own version of this OS on the same day.
https://www.notebookcheck.net/OnePlus-allegedly-plans-to-release-Android-10-on-the-same-day-as-Google.433433.0.html