Did they have to translate it into another language first?
Chinese Telegraph Code
That certainly seems harder than simply learning to translate letter by letter but I haven’t learned to do either, let alone both, so I could be wrong. In some ways, it kind of sounds like translating into another language- I could see an experienced operator knowing certain words and phrases without having to look in the book, that are either commonly used or of enough importance to be strictly memorized, but any unfamiliar statements and they would have to check the codebook. Which is exactly what you would expect from someone learning a language, knowing common or important phrases but needing to look at their translation dictionary when new words came up.
An experienced radio operator (in English) would be able to ‘read’ a morse signal from another experienced operator much as we ‘read’ a stream of words. There are also the ‘Q’ codes which translate as common phrases. No doubt the Chinese, and presumably all countries that don’t use the alphabet, have something similar - maybe an extended set.
Using Morse code for the digits seems an excessive waste of effort. Just adopting a simpler code that just encodes numbers would get you a four times improvement in information density. Let alone looking at an appropriate coding that took frequency of use into account.