I find that 'proof' tenuous?
The Henrichs article says:
(7) The doctrine of the “True Prophet.” The Pseudo-Clementines and Elchasai coincide in that they propagate the cyclic incarnation of the
True Prophet. For Elchasai, however, the series of incarnations did not culminate in Christ, but included Elchasai and continued even
beyond him. The Cologne Codex has a clear reference to this doctrine. Some of the baptists were so impressed by Mani's performance as a theologian that they regarded him as the True Prophet and the incarnation of the Living Logos. This doctrine, which lies at the root of
Mani’s own conception of his apostleship as the concluding stage in a series of incarnations, forms, in combination with the docetism of
Marcion and Bardaisan, the basis of Mani’s christology."
But the text goes on:
"These parallels are overwhelming. Henceforth, the fact that Mani grew up in, and was influenced by, Jewish-Christian baptists must be
reckoned with. This new insight into the religious environment in which Mani had lived for twenty years is quite a revelation in its own
right and will provide fertile soil for future studies.
"And yet the early history of Mani’s baptists is totally in the dark. The moment we consider their origin, we indulge in speculation, a tendency which has proved particularly dangerous in these studies. Nevertheless, it should be pointed out that the Elchasaite connections of the Babylonian baptists may be secondary and superimposed, perhaps through the adoption of Elchasai’s book of revelation, on an original Palestinian substratum. Two factors support such a hypothesis. In the first place, there is strong evidence for missionary activities of Elchasaite groups, in the late second and early third centuries, in places as different as Coele Syria (Apameia), Rome, and Palestine. It is conceivable, therefore, that another wave of this missionary tide reached southern Babylonia and mingled with existing currents, thus producing the special blend of Mani’s baptists. In the second place, Mani’s baptists led a communal life in isolated villages and emphasized manual labor, especially agriculture. There is no recorded precedent for such a form of organization in Jewish-Christian sects. But there are the Essenes and the Qumran sect of Palestine which provide analogies."
(Mani and the Babylonian Baptists: A Historical Confrontation, Albert Henrichs, pps 54-56, emphasis mine.)
Henrichs uses the term 'Babylonian baptists' throughout. Ebionites are mentioned only once, as being vegetarians, as were the Manichæan 'elect' – the 'hearers' were not under the same rule. Yet the Elkasaites were not vegetarian.
Where problems arise is highlighted above, and as primarily-gentile Christianity changed over the first centuries, so did the doctrines of the Jewish-Christian groups. To assume they remained pure whereas others did not is a step too far and is belied by what records we have.
No original doctrines survived, whole and entirely unchanged, from their founding.
It's more than likely that 'Ebionite' ideas contained in the pseudo-Clementine literature is the result of 'Ebionite' redactors, whilst clearly elements of the literature contradict what we assume to be early Ebionite belief.
In the lack of firm evidence to the contrary, I would suggest that the "Ebionite ideas of the cyclical successions of revelators" is not authentically Ebionite, if indeed ever part of their doctrine, but rather a later incorporation under the influence of Elchasaite and Manichæan groups, if not an erroneous assumption attributing to them ideas which were not theirs by their patristic critics.
to fourth century proclamations?