Onyomi means “sound reading,” of a kanji, an imitation of the Chinese pronunciation. The opposite is kunyomi, which means “interpretative reading,” which means they found an existing Japanese word that matches (to one degree or another) the kanji.
It doesn’t make sense for any Japanese people to have onyomi last names, because that would mean they didn’t have surnames before kanji was adopted. They would just find kanji that would match their surnames in meaning. Indeed, the overwhelming majority of Japanese surnames are not onyomi. But strangely enough, some are. The ones I’ve encoutered: