Have you actually contributed to the Go standard library?

Yes, there are people who don't work for Google and can +2 on changes, but you still need 2 Google employees to at least +1 on your change before it can be submitted. This is mentioned in the Contribution Guide [0] and is enforced by Gerrit.

> Finally, to be submitted, a change must have the involvement of two Google employees, either as the uploader of the change or as a reviewer voting at least Code-Review +1. This requirement is for compliance and supply chain security reasons.

[0] https://go.dev/doc/contribute

When someone says demonstrably, I expect they would follow up with said demonstration

Then it should be easy to find examples of changes going into the STL that have not had the approval of multiple Googlers.