The sound is exactly kw, so kumkwat is an honest transcription. It is also an impossible English spelling. Norman scribes replaced the Old English cw with qu in the twelfth century, and the substitution was total: queen, quick, quiet, quench. English has kept no ordinary word that writes the kw sound as kw. When the fruit name arrived from Cantonese it was handed to the same convention, and gwat became quat.