Most of internal Google services don't run on Google cloud. Was true as of a couple of years ago and I haven't heard differently from my Google friends recently.
Just reclassify Borg as Google Cloud Internal Edition and boom, everything runs on Google Cloud. This sounds stupid but it's no worse than what MS and IBM have done. Heck, when you are your own customer maybe you could even double-count some revenue...
FWIW... most of Amazon's internal infrastructure was NOT run on AWS as of a couple of years ago. The idea that AWS was built to support Amazon is a myth. In fact, most of their own IaaS was a very traditional data center built on converged infrastructure. This is why Amazon was up and running when AWS would have a large outage.