This has nothing to do with Google Play services. Replacing the google services with another proprietary crap would be nothing (and cost relatively nothing) compared to building an entire operating system almost from scratch. This is a dick measuring contest. Although the chinese dick is shiny and impressive in this regard so far.
Plus, even google is playing with the idea of dropping linux, with the Fuchsia project.
In China is easy. Once you give some millions to tencent/Alibaba/Baidu/bytedance (or having the government force them to do so) for porting their apps, it's all a Chinese citizen needs.
There are many apps that are actually "mini apps" (a WebView inside tencent WeChat)
When you enter China you need to register with the customs using said "mini app" in WeChat
90% of users exclusively use WeChat, alipay, Baidu, taobao, amaps, douyin (TikTok), xiaohongshu.
Maybe some could find useful to also have Weibo and pinduoduo (temu malware) but the "must have" apps are really a handful, it's not like in the west
Tizen was written from scratch. From what I understand HarmonyOS is Android... at least it was forked from Android. oh shit, I talked before reading.. sorry for that! :-D