It is possible that the author was trying to portray a Roman-looking Emperor. We have very few reliable illustrations of Frankish equipment from this period (https://commons.wikimedia.org/wiki/C...ib_Bibl.fol.23 https://commons.wikimedia.org/wiki/C...Codex_PER_F_17 ), and those which go for idealization tend to show Roman-like equipment, possibly under Byzantine influence ( https://commons.wikimedia.org/wiki/C...uve_-_BNF_Lat1 ).

There's also the fact that certain Roman art pieces like Trajan's Column were well known, and some have identified their influence on Medieval art (some e.g. see the Bayeux Tapestry as a reelaboration of the Column).

The Latin word for armlet doesn't say anything about length. Manicae could even be handcuffs, or sleeves covering the whole of the arm. The term is also used in the testament of Everardus, one of Charlemagne's dukes, likely also describing an armourpiece ( Bruniam unam, helmum unum, Manicam unam. ) So we can suppose that Charles wasn't the only one using those in battle, and that they really did exist, at least for the elite.