Think carefully about this: Does the non-existence of a historical Ebion imply the existence of a mythical Ebion dwelling in a sublunary supernatural domain?
No, it doesn't.
Still, it's curious you say that because according to Epiphanius
" They say that Christ was not begotten of God the Father, but created as one of the archangels ... that he rules over the angels and all the creatures of the Almighty, and that he came and declared, as their Gospel, which is called Gospel according to Matthew, or Gospel According to the Hebrews, reports: "I am come to do away with sacrifices, and if you cease not sacrificing, the wrath of God will not cease from you."
(Epiphanius, Panarion 30.16,4-5) "
ETA The Ebionites didn't assert the existence of a historical Ebion. They asserted the existence of a non-divine, non-virgin born, historical Jesus. The Ebionites are another piece of evidence, and a significant one (where of course all the evidence is weak), in favour of Jesus having originated as a credible historical figure within the Jewish messianic movements.
Perhaps I'm reading my source* incorrectly, but I have the impression they were perceived differently, at least by Epiphanius, than you've represented them. And the fact is we have have their texts via Epiphanius, correct me if I'm wrong.
http://www.jewishencyclopedia.com/articles/5411-ebionites agrees with you, however.
I wonder why Epiphanius wrote of their beliefs as he did?
Irenaeus didn't love them either
"2. Those who are called Ebionites agree that the world was made by God; but their opinions with respect to the Lord are similar to those of Cerinthus and Carpocrates. They use the Gospel according to Matthew only, and repudiate the Apostle Paul, maintaining that he was an apostate from the law. As to the prophetical writings, they endeavour to expound them in a somewhat singular manner: they practise circumcision, persevere in the observance of those customs which are enjoined by the law, and are so Judaic in their style of life, that they even adore Jerusalem as if it were the house of God."
http://www.newadvent.org/fathers/0103126.htm
They were high on Eusebius' hate list, too
"1 The evil demon, however, being unable to tear certain others from their allegiance to the Christ of God, yet found them susceptible in a different direction, and so brought them over to his own purposes. The ancients quite properly called these men Ebionites, because they held poor and mean opinions concerning Christ. "
http://ancienthistory.about.com/library/bl/bl_text_eusebius_iii27.htm
Very, very curious Epiphanius claimed something so divergent for these people's views.
In any case, the beliefs of some cultists hardly represent evidence of an historical Jesus, but rather data for a belief system at that time, IMO.
*
http://www.earlychristianwritings.com/text/gospelebionites-panarion.html