First of all, don't take it so literally. Exaggeration is allowed for eulogies, and I assume for an epitaph as well. Consider the fact that Rambam himself called his masterpiece Mishna Torah, the rabbinic name for Deuteronomy, he seems to have made this connection himself. But especially let's remember that Moshe Isserles has this same quote on his Epitaph, so let's not worry too much about this.
That being said, the way I've seen it explained was this praise was said as far as his being a Marbitz Torah, that means someone who brings Torah to the masses. This has nothing to do with intelligence or any other measurement of personal greatness.
This of course can be debated. Some possible contenders would be King Chizkiyahu and Rabi Akiva. But we can argue that Chizkiyahu did not personally teach Torah to the masses, he instituted a system where people would learn Torah, but he was not the teacher, see Sanhedrin 94b.
Rabi Akiva, although being a teacher and having the unnamed sources in Torah Shebaal Peh coming through his students, according to his opinion(Sanhedrin 86a), did not reach the masses personally. Rather it was his students that did. Rabi Akiva taught the five through whom Torah was refounded (Yevamos 62b). (As I'm writing this, another level of understanding suddenly hit me what the loss was when his thousands fold group of students, who he seemingly did teach personally, all died.)
There are other personalities in Nach and Shaas who we can debate about as well, see for instance Bava Metzia 85b about Rabi Chiya, but that is the same situation as Rabi Akiva. Also Sanhedrin 21b concerning the greatness of Ezra: 'Ezra was worthy to have the Torah given through him, had not Moshe preceded him'. This is probably the strongest question I can think of right now on that Epitaph (@Mike pointed this out in the comments as well), but again we can try to say the case of someone who renewed the Torah to the masses, upon whom praise is showered as being worthy of giving it originally had he not been preceded by another, is different than someone who actually gave something original to the masses.
Also along these lines is the fact that Rambam's original, personal work was so widely received. This echoes Moshe Rabbeinu's receiving and giving of the Torah, whereas other works of law, namely the Mishna and Talmud were compendiums of many opinions. The works of the Geonim, although arguably personal and not communal, were not as widely accepted by the masses outside of the towns where their Yeshivos were. This is obvious from some letters of the Geonim, and the masses remained extremely unlearned.
