That 91 year old also likely had family, colleagues and other acquaintances who ...

bArray · on Aug 10, 2016

Exactly and in the UK military (or military like structure), it's easy to get in if you have relatives in.

Also, just because the originator may be dead, it doesn't mean the work won't have knock on effects.

And finding a hole in security for old material may trigger a search through all newer ciphers - something I'm sure they don't want.

There are plenty of reasons, correct or not, why they may wish to keep this a secret.