Why does it matter how much of the catalogue is high bitrate? That (IMO) is not the primary benefit of support. Quite simply, Apple Music is a huge service by number of subscribers, and providing support for it would be of utility to a large number of current Volumio subscribers, and would be a potential selling point towards adding more.
Thus far, I’ve seen nothing to indicate that Apple is pushing away developers who wish to integrate with AM, small or otherwise.