It was (for better and for worse) the #1 reason to adopt GCP for our main cloud platform, and we're not alone.
It's has seen immense adoption that it wouldn't have had if it hadn't been in a neutral foundation. The work was definitely not done by Google alone (~30%).
IS GCP "still" #3 ? Yes. Is it because they didn't get a tighter grip on k8s and try to use it as a competitive advantage ? absolutely not. Nobody would have touched k8s with a 10 foot pole in that situation.
The strength of k8s is in its eco-system and that wouldn't have happened without the CNCF.
And I'm saying this as an early adopter of k8s, it's insane for the Google brass to see anything other than an immense success in the handling of k8s, at least from the perspective of how much money it made Google (indirectly)
It's not because of K8S strategy that GCP is #3. It's just because MS & AWS are better. PERIOD. Better at service, better at marketing, better at some product design sometimes, better at abusing their current dominant position (hehe it sucks doesn't Goog ? heh? Doesn't it ?)