I'm aware that RTL Japanese was a thing. In fact, what you have described is not really RTL; its inline direction is top to bottom and its block direction is right to left [1] while what we typically refer to RTL is about the inline direction. RTL Japanese is actually a specific variation of this TTB/RTL writing where the line itself only consists of a single character. Very rare (typically signages), but it did exist.

But that doesn't mean your RTL text should be preemptitively reversed in the memory, which is exactly what the exercise asks you to do. Historically this was the case for some early character encoding, where the reversed text is called the "visual order" opposed to the "logical order". This caused enough problems that Unicode is now primarily the logical order only, except for a few scripts where the historical visual order is retained (e.g. Thai).

[1] See https://www.w3.org/TR/css-writing-modes-4/#text-flow for more information.