Diego Garcia military base apparently know about the earthquake an hour before it happened. They did not report it to the affected countries.

At this stage of scientific advance it would have been impossible for Diego Garcia base or anyone else to "know about the earthquake an hour before it happened". Just can't be done.

What can be known, and what they apparently did know about, would have been the earthquake when it happened, and about the implications so far as the resulting massive tidal wave (tsunami), well before it would have reached Diego Garcia, or other places.

However the information they had about this would have been the same as that reaching all kind of other places all over the planet with immediate access to seismographs. Clearly there was a failure to get this information out in the right way to the people who were in danger, and enormous nunbers of people died as a result of this failure. Just where the blame lies for collective is far from clear, however pointing the finger specifically at the people on Diego Garcia is not really justified.

And my basis for saying this is BBC coverage over the period. (Of course that base shouldn't be on Diego Garcia in the first place, because the place was stolen from the inhabitants by the then British Government, at the insistence of the then US Government, with the inhabitants being deported to other British posessions. But that is another matter.)