I have to agree with him. I had to fix an app for a Huawei Honor 8x phone the other day. As the phone doesn't have Google Play Services and doesn't offer it in the app store - it's a huge pain using it. Enthusiasts can sometimes hack Google support on to an AOSP edition, but as app developers, we have to support what every day customers can do.
There are so many apps that won't work on it at all since Google has been steadily replacing AOSP services like location with Google Play proprietary versions. If you go to the open version of location services, you'll see there's a huge note at the top that you should really go use Google's version:
https://developer.android.com/guide/topics/location
And any app that requires that, isn't going to work on Android releases that don't have Google's blessing like Huawei.
The point regarding Play Services is valid, and if the parent comment had focused on that I would not have objected. But that doesn't mean AOSP is a "very small part" of Android or that it isn't open source.
The flip-side of your experience is that it highlights the existence of a large number of Android devices based on AOSP that don't run Google apps, particularly in the PRC. Aside from the Chinese market, you have a major company like Amazon selling millions of devices with their own AOSP fork on them. On an individual level, I can install Lineage OS on my unlocked devices and use apps from F-Droid as well as many others.
Play Services does have a significant foothold in many users' experiences, which bears discussion, but it doesn't invalidate AOSP. It is Android's open source nature that allows for forks and for apps to be shared across the Android ecosystem much more readily than programs developed for an actual proprietary operating system such as Windows.
At the same time, we're getting articles talking about how Microsoft is now an "open source company" because their proprietary operating system can run Linux and some open source apps: https://arstechnica.com/gadgets/2019/05/microsoft-the-open-s...