True for objects, not true for many static environment things, or where baked AO is an optimization because SSAO is too computationally expensive / low quality. For lightmaps, Unity defaults to UV2 and creates these, but also assumes the artist might want UV1 for a detail map since that is common these days. For most of us in the web, optimizing for one UV set + lightmaps is plenty.
I just with glTF natively supported light maps out 