This article is based on historical FACT!
Go a watch the movie “Midway” starting Charlton Heston, et al. The whole outcome of the Battle of Midway hinged on correctly positioning the inferior American Fleet.
The writer is correct - once you break a code system you can then “read” any and all mails using that code. Another point made in the same movie.
But, most importantly once you break a code and analyze it your job of breaking other codes becomes much easier.
How?
Developing a new code takes time and effort. o once you field a new code it is, unfortunately, much easier to field variations of that system for other users/replacements. While you may not be able to read entire messages you will be able to make out parts of it, Those parts canbe pieced together to fully break the code system. Again, another point made in the movie.
There are ways around this but they are slow, difficult, and greatly disliked by those non professionals (aka politicians) that have to use them.
Been there, done that, and had numerous t-shirts during my almost 40 years exposure to the DOD, codes, and other esoteric subjects.
The writer is correct - once you break a code system you can then “read” any and all mails using that code.