This is like saying that all the countries in the world that speak English are England, that's not how it works. Countries are not defined by the language they speak, and the language they speak most definitely varies over time.

Moreover you're picking a date hundreds of years into the eastern empire's existence and declaring "because they switched official languages hundreds of years later, they were always Greek". Time doesn't flow like that.

Moreover the roman empire, pre-split, spoke quite a bit of this language (well, Koine Greek, the predecessor to Romaic). Including in official communications (particularly in the east), and at least one emperor's private diaries [1].

[1] https://en.wikipedia.org/wiki/Meditations