Google to acquire Apigee 10 years ago
It makes no mention of using it on Google's APIs at all.
Within the blog it elaborates on what they're doing by saying:
Google cloud customers are already benefitting from no sys-ops dev environments, including Google App Engine and Google Container Engine. Now, with Apigee’s API management platform, they'll be able to front these secure and scalable services with a simple way to provide the exported APIs.
As always, we'll make sure that these capabilities are available in the public clouds and can also be used on-premises.
If you click on the link and read the article it will give information behind the headline, possibly answering questions the headline brings up.