It sounds like the solution was always left as an exercise for the reader in the day-to-day dealings of the group even among the members: "Each participating country was supposed to perform its own decryptions. This is common practice in the intelligence community in order to prevent being fed cooked-up information." and "The cryptanalytical part of the cooperation involved exchanges of algorithms used in various (deliberately weakened) cryptographic devices used by target countries. It was then left up to the Maximator participants themselves to find out how to exploit weaknesses in the algorithms of these devices. Such exploitations are also called ‘solutions’."

That was my read as well.

That said, you could spin it another way: the UK had enough good will and political capital to lean on its European neighbors for a bare-minimum amount of help. "We can tell you how it works, but it's on you to figure the rest out".

Naughty kid not paying attention is also relevant -- you've only got so many hours in the day and so many hours to map out threats. This was still the Cold War and nuclear holocaust was a bigger priority than distant Latin American dictatorship.

I think it's about plausible deniability. If they only told gchq about the design of the algorithms, they're not saying they know how to break them or even that they're designed to be breakable.