Itīs there to symbolize how the Byzantine empire relied heavily on foreign troops and mercenaries, and how they lagged behind in the technological development after the high Middle ages.
The Turks, on the other hand, historically became more advanced than the Byzantines after that time, which makes it logical that their late-game heavy cavalry can outmatch the Byzantine ones.
The Byzantines still have good heavy cavalry though - the Latinkon and Cataphractoi are very capable.
My greatest concern is that they should have had an infantry unit more similar to legionaries (at least in appearance).
This is because the Byzantines historically relied on a similar type of legionary-like swordsmen for their main lines (or rather, the part of the main line not made up of foreign mercenaries), and for RP-purposes

When I play the Byzantines, I want to imagine the Roman Empire being reborn, and use roman-style tactics on the battlefields - unfortunately, the closest thing to a unit that resembles legionaries in their roster is the Byzantine Infantry...