I think you are getting mixed up about hosts and classes of hosts.
There is the HID (Human Interface Device) class, this handles things that are relatively slow, like mouse movement, keyboard input, joysticks and so on. There are lots of different classes see:- USB defined classes
A pdf giving details of the USB MIDI class can be downloaded by clicking on this link Click to Download PDF about the MIDI USB class
There will be a lot of detail here you do not need / wish to know. But just take the point that there are different classes of USB device, and a HID interface is not a USB MIDI device.
I have made the Hobbytronics board into a universal USB to MIDI Host. I have put this into a box and used this to implement a host on a project that is built around an Arduino Micro board. Note that the Arduino Micro board is not capable of acting as a host.
Hence your assertion that:-
Is not in fact true.
I hope that this clears things up for you.