This is so good that I wondered if it's fake. Really impressive results from generated spectrographs! Also really interesting that it's not exactly trained on the audio files themselves - wonder if the usual copyright-based objections wild even apply here.
regarding those usual objections, i'd argue that a spectrograph representation of a given piece of audio is just a different (lossy) encoding of the same content/information, so any hypothetical objections would still apply here.
In theory the tone quality is not an objection here. When it sounds bad it's because it's 512x512, because the FFT resolution isn't up to the task, etc. People cling to very inadequate audio standards for digital processing, but you don't have to.