You've got the reasoning backwards. Themistogenes is an individual whom Xenophon cites in the Hellenica, claiming that he wrote a work describing, as the Anabasis does, the march of the Ten Thousand. Themistogenes is otherwise unknown. The Suda, in its article on Themistogenes, claims that Themistogenes wrote not only an Anabasis of his own but "καὶ ἄλλα τινὰ περὶ τῆς ἑαυτοῦ πατρίδος," "other things concerning his own homeland." The Suda is unique in that it is the only of our four textual allusions to this Themistogenes that claims that he wrote anything other than the work that Xenophon claims for him in the Hellenica. Our other two, Plutarch and the Byzantine scholar Tzetzes, both definitely state that Xenophon wrote the Anabasis, but give different reasons for why Xenophon attributes it to Themistogenes. Tzetzes, whose testimony is late and, in context, rather strained, claims that Xenophon wrote the Anabasis for Themistogenes, who was his lover, and that he initially put Themistogenes' name on it but that later scholars and publishers put Xenophon's name back. Plutarch's testimony is a little bit more realistic, and Tzetzes may be elaborating on Plutarch. Plutarch says that Xenophon attributed the Anabasis to Themistogenes in order to remove himself a step away as its author and lend more credence to his actions as described within the work.

If Themistogenes was a real person and actually wrote an Anabasis (whether his own or identical to the work now attributed to Xenophon) then we have a bit of a problem textually. Hellenica 3.1.2 cites this otherwise unknown "Themistogenes" regarding the events in Asia with the Ten Thousand. The assumption is that Xenophon would more reasonably have cited himself, since he was a literal eyewitness to the events and had written about them, but he does not. This has led Brownson, the old editor of the Loeb, to claim that the Anabasis was written after the Hellenica. But most scholars date the Hellenica as firmly after the Anabasis. Why, then, did Xenophon not cite himself? One might say modesty, but again Xenophon was probably the best literary authority on the Ten Thousand alive. There are quite a lot of theories, none of which plausibly postulate that an individual called Themistogenes actually wrote the text of the Anabasis--it simply bears far too many traces of Xenophon and his style, and the lack of any attestation of such a person is extremely suspect. Breitenbach way back in the nineteenth century (I think he was the first, in any case certainly by the early twentieth century it was a common view) proposed that there was no such person as Themistogenes, and that the name is simply a pseudonym for Xenophon to make his work more credible, as Plutarch asserts. By this proposal Xenophon either republished the text with his own name on it when it became unnecessary to hide behind a pseudonym or that it was common knowledge that Xenophon wrote it (how could it not have been?) and that later editions simply put Xenophon's name on it. The problem with this suggestion is that other than the brief testimony we have in the Hellenica and the handful of references to that passage we have no evidence to suggest that the Anabasis was ever published under anyone's name but Xenophon's. Still, most editors and scholars accept some version of Plutarch's statement that Themistogenes is just Xenophon. There are problems with that, though, although not to the degree of supposing that Xenophon didn't write the Anabasis. More robust attempts have been made to recover the existence of an individual called Themistogenes who may have written his own Anabasis. After all, the author Sophaenetus, mentioned by Xenophon in the Anabasis as one of the leaders of the Ten Thousand, apparently wrote his own Anabasis, if the sole, much later testimony of Stephanus is genuine. There must have been at least two Anabaseis floating around, and it's perfectly reasonable to suppose that another one, written by this enigmatic Themistogenes, might have also existed. Recent questioning about when the Anabasis was published and for what reason (in particular why Xenophon writes his narrative entirely in the third person, which was novel for this sort of thing) have led to some doubt about when the Anabasis existed in relation to the Hellenica and whether Xenophon would have cited himself if it had already been written. Recently Tsagalis has suggested that Xenophon is referring to an abridged version of his own work compiled by Themistogenes, which would have contained the first four books. But that view is untenable in light of what Hellenica 3.1.2 actually says, namely that Themistogenes' text apparently described how ἀπεσώθησαν οἱ Ἕλληνες ἐπὶ θάλατταν, "the Greeks were saved at the sea." Tsagalis suggested that since the Ten Thousand reach the sea in Book 4 that's as far as Themistogenes' abridgment went, except that Pelling more recent still has pointed out that ἀπεσώθησαν, "they were saved," is inconsistent with the situation in Book 4, since the Ten Thousand are still very much up the creek when they reach the sea. There have been attempts to interpret Hellenica 3.1.2 as an interpolation, which probably would have happened some time before Plutarch got access to the text (and explains Plutarch's incredulity, since Themistogenes would be a made-up person). But why would an interpolator attribute the Anabasis to somebody other than Xenophon, especially if Themistogenes did not exist and it would have been plainly obvious (at least to Plutarch) that this did not belong? In a very old article Prentice suggested that the text is corrupt and that instead of Θεμιστογένει τῷ Συρακοσίῳ γέγραπται, "it was written by Themistogenes the Syracusan" Xenophon's text should actually read θεμιστῶς ἑνί τῶν Κυρείων γέγραπται, "it was written dutifully by one of those with Cyrus." Which means that Xenophon would be citing himself, as seems logical, without actually naming himself. The text in this view would have become corrupt sometime before Plutarch (Plutarch was centuries later). The idea pops up now and again sometimes, although I really have difficulty accepting it. Θεμιστῶς is a really weird word to have there and doesn't seem to contribute much, and I'm not fully convinced by the paleography arguments that argue that the scribal error would have been relatively easy there, the lines just seem so totally at odds with each other than during recension the error in the manuscript surely would have been spotted by anyone paying any attention to what the text actually said.

In any case there are no prevailing views that Xenophon isn't the author of the Anabasis, rather the controversy rests in whether Themistogenes is a pseudonym for Xenophon or whether there really was a parallel Anabasis written by some dude from Syracuse.