I know android itself can support multiple archs, but does a (random play store app with native library) support them or even test them all?

Most Play apps have no NDK code so they work fine.

Not claiming it's a representative sample, but my phone has 160 subfolders underneath /data/app (i.e. user-installed/-updated apps), of which after a quick count 52 include native libraries.

I don't even have that many games on my phone, so those only account for six of those apps, with the largest fraction being various multimedia apps (camera, video player, image editor, music player, even my e-book reader…)

The top Play apps and games use native libraries. Here's a CppCon talk that mentions that 75% of the top 100 Play apps and games use C++.

https://youtube.com/watch?v=2Y47g8xNE1o&feature=shares