As well, as any honest engineer knows, new tech is rarely reliable and bug free. You may adopt it for other benefits, but assurance is generally not one of them. So if lives depend on something, you may keep using things that have been proven reliable.
It’s even harder to train new people in Old technology. Just write a code base in Fortran And see how hard it is to find a developer.
I’m quite confident you could send messages all day using the methods of Ancient Rome without being ever detected.
My thought is, that new people are trained on modern technology (how to acquire and set up a secure laptop OS or how to configure a smartphone), and older employees still "run" on the old technology.
They seemed to have communicated a lot with radio and coded messages. They also used some Windows software to decode some of those messages. And exactly there they made a mistake and some messages could be restored.
It seems like they only got very limited recurring training after their initial training in the 90s. So they might have had very limited IT opsec knowledge.
The analog radio technology is also far from perfect. In their case the neighbours became suspicious, because they never opened the door at specific times, probably when their transmissions were scheduled. They also sent some radio transmissions from a nearby hill, that might have played a part in their capture.
I'm convinced that some encrypted messages over a commonly used messenger or email provider would be way more secure. They would just disappear within billions of other encrypted messages.