"Google hired Thompson to create a new language, Go. But Google also requires all of its recruits to pass a language test. According to Thompson, he hasn't quite got round to it yet - and so can't submit code."
It's not and it wasn't about any C++ "certifications." He was involved in the creation of a new language. For the new language there could have not been any existing certification. And as far as I know, the initial implementation language of Go was not C++ but C. In creation of which was Thompson also directly involved. And as far as I know the people worked on the parts of the code before Google and these parts of said C code already existed before Google (Go had some dependencies on code from https://en.wikipedia.org/wiki/Plan_9_from_Bell_Labs and https://en.wikipedia.org/wiki/Inferno_(operating_system)).
So it was indeed as absurd as it could have been. He was a part of the team that directly invented both the language and the style guide of the language, but some bureaucratic exam was expected from him to submit the code to the source control system.