I read a book on that incident a number of years ago, Smalls' book was published in 1988, but I thought I had read a book around 1980 on the topic. At the time they had to cover it up because of preparations for Overlord. But in subsequent years there was no reason not to come clean on the tragedy.
We tend to be a bit over zelous sticking with our 50 and 60 and 100 year 'offical secrets' rules. That said the slapton sands incident has been the subject of many TV documentaries even in the absence of official acknowledgement up to know. There were that many witnesses there is little doubt it happened.
Tragic - but that's war. My own grandfather told me he saw more men lost in training accidents than he did in combat.