Isn't byzantine literally just eastern Rome? I don't get waht Caesar is trying to say
Caesar's line is basically saying that while the Byzantine Empire/The Eastern Roman Empire is Roman, it's so culturally different from Caesar's Rome (you know, back in the BC times) that it can be seen as a completely different civilization, but he's amused that despite hundreds of years passing and multiple cultural shifts, people like Constantine were still considering themselves Roman Emperors.
His wording is a little bit underhanded. "Kawaii yatsu da" isn't exactly the politest way to refer to a peer, but it's moreso overly familiar and arrogant in a Caesar-style way than actively insulting.
Basically he's saying - "Byzantium is so different from Rome, but he still considers himself a Roman emperor. How cute."