Oh my god...
I didn't realize I needed to copy the entire function CDRMV provided. I thought it was a built-in function like GetMapSize().
When it didn't work, I headed over to Github to look at more examples. Of course, they all used getPlayableArea(), which I copied.
Then, it finally occurred to me CDRMV meant for me to copy the entire function. Except, I forgot I'd switch my syntax to getPlayableArea(). Only took an hour of troubleshooting to see it.