Actually, no, they can't. Aliyun is an AOSP fork without Google Play services (much like Amazon's Fire OS). Google stopped Acer from releasing a phone with the OS, as it violates the Open Handset Alliance terms.

Alibaba: "Our partner was notified by Google that if the product runs Aliyun OS, Google will terminate its Android-related cooperation and other technology licensing with our partner"

They're leveraging their extremely popular (market share %) OS + app suite (which OEMs can't unbundle) to effectively force OEMs into contractual agreements which forbid them from selling devices with competing Operating Systems. This is anti-competition.

Google aren't allowed leverage the popularity of their OS + app suite to stifle competition - it's textbook abuse of monopoly. Google knows that if these OEMs try to 'go it alone' without Google's Android (and all that's bundled with it), they'll crash and burn. Much like how Microsoft knew OEMs would accept IE being bundled, because going it alone (with Linux or their own OS) would not be viable.

As a footnote, do you know who manufactures the Fire OS (Kindle Fire) tablets? Quanta Computing. Members of the Open Handset Alliance (which include the majority of Android OEMs) are contractually forbidden to produce Android devices based on forks of the OS.

This is a fairly good read regarding what's being discussed in this thread.