Let's take a close look at the passage in Against Celsus. Origen begins by noting that:
Εἶτα σωρὸν καταχέων ἡμῶν ὀνομάτων φησὶν εἰδέναι τινὰς καὶ Σιμωνιανούς, οἳ τὴν Ἑλένην ἤτοι διδάσκαλον Ἕλενον σέβοντες Ἑλενιανοὶ λέγονται
He next pours down upon us a heap of names, saying that he knows (εἰδέναι) of the existence of certain Simonians who worship Helene, or Helenus, as their teacher, and are called Helenians
Clearly Celsus makes explicit mention of the Simonians and that they are related to a certain woman. These stories show up in Irenaeus and Hippolytus. Then we learn from Origen again that:
Κέλσος μὲν οὖν οἶδε καὶ Μαρκελλιανοὺς ἀπὸ Μαρκελλίνας καὶ Ἁρποκρατιανοὺς ἀπὸ Σαλώμης καὶ ἄλλους ἀπὸ Μαριάμμης καὶ ἄλλους ἀπὸ Μάρθας
Celsus knows (οἶδε), moreover, certain Marcellians, so called from Marcellina, and Harpocratians from Salome, and others who derive their name from Mariamme, and others again from Martha.
In each case the verb Origen uses is οἶδα (= to know) so the same kind of knowledge or familiarity is understood throughout. These are sects that Celsus specifically mentions in his work the True Word.
Yet Origen's remark about Celsus making an allusion to the Marcionites is very different and comes at the end of this section of sects familiar to the anti-Christian writer. Origen here offhandedly makes the comment after the reference to the female sects that:
Ἐμνήσθη δ´ ὁ Κέλσος καὶ Μαρκιωνιστῶν, προϊσταμένων Μαρκίωνα
He is aware (ἐμνήσθη) also of the Marcionites, whose leader was Marcion.
It should be noted that ἐμνήσθη here is usually translated as 'he makes mention of' or 'il nomme' in French but mimnḗskō literally means recall, bring to mind, remind oneself actively (purposefully); to remember, have in mind, "be mindful of."
Are we reading too much into the change of wording in the chapter? I don't know. Certainly Origen could have meant 'he also made mention of the Marcionites' (not Marcion though). Yet I get the sense at least as if Origen has been literally dealing with a section of text in the True Word which makes explicit reference to the manner in which Christian groups are formed around women and the Origen adds in his own voice that Celsus also 'has in mind' the Marcionites. The reason I say this is because all that follows in Origen's Against Celsus is a lengthy account of Celsus making reference to a Pauline sect where Celsus make reference to passage of Pauline scripture cited by this group.
It is worth noting that Origen never again references Celsus this way in the True Word. In Book Six however he alludes to his previous use of the epistle of Plato in the following manner:
Ἀλλ´ οὐδ´ ἐβουλήθη τὸ παρὰ Πλάτωνι ἐν ταῖς ἐπιστολαῖς λελεγμένον, οὗ ἐν τοῖς ἀνωτέρω ἐμνήσθημεν, περὶ τοῦ διακοσμήσαντος τόδε τὸ πᾶν ὡς ὄντος υἱοῦ θεοῦ παραθέσθαι
Nay, he would not even quote the passage in the letters of Plato, to which we referred in the preceding pages, concerning Him who so beautifully arranged this world, as being the Son of God
The question here isn't whether or not Origen names the individual letter of Plato. His use of mimnḗskō here only makes clear that he 'had in mind' the contents of the letters. The question of whether he names the letters isn't the point in discussion. I think Origen's allusion to Celsus's reference to the Marcionites is much the same. Origen is merely saying that Celsus 'has in mind' the Marcionites in what follows and throughout the discussion. Yet there is no proof that they are mentioned by name. Origen only knows that the opinions cited by Celsus 'fit' the description of the sect which Origen knows to be the Marcionites.
The point is that there is no reason to believe from Origen's description of the contents of the True Word that the specific name 'Marcionite' was to be found. The beliefs of the Marcionites are take to represent a large portion of Christians - perhaps even the majority. One might even wonder if the reference to the 'Marcellians' which is certainly explicit in the True Word is the historical source of the Patristic reference to the Marcionites. As mentioned, Jerome identifies Marcellina as a Marcionite which at first strikes us as odd because the Hypomnemata of Hegesippus identifies Marcellina as a 'Carpocratian.' Yet notice that the two names stand side by side on the page in Origen and 'Harpocratian' (= 'those of the Horus the child') have already also been corrupted into 'Carpocratians.'
All the later Church Fathers who cite the existence of a sect called 'the Carpocratians' are also familiar with Hegesippus's text. One may be tempted to say that Hegesippus is the source of the error here (Origen plainly says that he has never met a Carpocratian). Why can't the same thing be true with the 'Marcellians' (again not specifically identified in Celsus as 'of Marcellina' Origen does that for us)? This is the only way in fact that we can begin to explain why Jerome thinks that Marcellina was the first Marcionite missionary to Rome. In other words, that Marcellina wasn't explicitly identified here as a 'Harpocratian.'
The name Μαρκελλιανοί could also have been used to identify a group centered around a male named 'Marcellus.' The Μαρκελλιανοί are indeed identified a heretical sect 'of Marcellus' in the fourth century.
