They quote a study by Akamai, a CDN company. Of course, if you run a CDN then you don't like products that help reduce latency when you don't have a CDN...
Server push is most useful in cases where latency is high, i.e. server and client are at different ends of the globe. It helps reduce round trips needed to load a website. Any good CDN has nodes at most important locations so the latency to the server will be low. Thus server push won't be as helpful.
CDNs loved the idea of HTTP/2 push. It's a complicated low level feature. To make it work, you'd need to figure out what to push, and the ideal way to prioritise and multiplex those streams to optimise for first render. CDNs are in the business of knowing this stuff better than anyone else, yet they still couldn't make it work.
Remember, most sites using CDNs still go to the root server for HTML and other no-cache content. It's only the more optimised sites that figure out how to deliver those resources straight from the CDN without consulting the end server.
Server push is most useful in cases where latency is high, i.e. server and client are at different ends of the globe. It helps reduce round trips needed to load a website. Any good CDN has nodes at most important locations so the latency to the server will be low. Thus server push won't be as helpful.