The arms, equipment and impact of Late Roman Clibanarii

[Renatus]

I was particularly thinking of Equites Scutarii and Equites Promoti as comitatensian-grade armored cavalry.

There are several units of equites catafractarii named in the Notitia, as well as an equites promoti clibanarii and a scola scutariorom clibanariorum. I, therefore, think that the units you mention could perfectly well have been designated as catafractarii, if that is what they were.

Ammianus says a (singular, one) type of weapon when referring to the cataphractarii and the sagittarii.

Let me see if I understand your argument correctly. Ammianus mentions cataphractarii and sagittarii in the same sentence and also in the same sentence refers to a single genus of weapon (or, perhaps, soldier depending upon how you translate armorum). You conclude from this that cataphractarii and sagittarii are the same thing or, at least, similarly armed. I do not think that this follows. Ammianus appears more likely to be referring to two different types of unit, the cataphractarii and the archers, and seems to be separating them further by inserting the verb erant between them. Logically, therefore, formidabile genus armorum relates to the last type of unit mentioned and would indicate that the archers were particularly effective.