They can ship Google certified phones based on Android forks as long as they pass CTS[1]. You don't really think the Galaxy phones are built on top of unmodified AOSP, do you? Many[2] manufacturers[3] provide[4] SDKs[5] for their phones on top[6] of the base Android SDK.
This point doesn't support your first sentence then because this policy of the Open Handset Alliance existed before Google was in a position to bully, before Android had any market share at all. https://en.m.wikipedia.org/wiki/Open_Handset_Alliance?wprov=...
While it might not be an official requirement, being granted a Google apps license will go a whole lot easier if you join the Open Handset Alliance. The OHA is a group of companies committed to Android—Google's Android—and members are contractually prohibited from building non-Google approved devices. That's right, joining the OHA requires a company to sign its life away and promise to not build a device that runs a competing Android fork.
Besides, even in 2008, what other OS were OEMs going to license to compete with the iPhone? Windows Mobile?
This is what Google got fined for.
https://amp.theguardian.com/business/2018/jul/18/google-face...
The second anti-competitive behaviour was preventing smartphone manufacturers from running competing systems that had not been approved by Google.
For instance, Amazon wouldn’t have been allowed to manufacture both Fire phones and phone that were certified by Google as an Android phone.