I was talking sustain as feedback is an entirely different thing as you say.I think that there are two phenomena occurring there:
1) The sustain is increased by the vibrational energy that returns from the speaker to the guitar body. That is what I get with my Bluetooth Speaker: eternal sustain (without harmonics) that can even solve the problem with guitars that have a nasty dead note.
2) When that feedback loop gets saturated, harmonics are produced. That is what normally guitarist mean by "feedback". e.g.: Joe Satraini "Flying in a Blue Dream"