I'm a bit confused. Is it actually calling home? That seems kind of unlikely, given that a) that seems pretty egregious for a library like this, and b) they said this doesn't affect the application at all beyond consuming "a few bytes".

Could it instead be just that Google scans Google Play apps for a string in the binary that matches the Flatbuffers version string format? That seems more likely given what the README does say about this. And it also seems more useful in general; Google would benefit more from knowing how many applications use the library than knowing how popular these applications are.