But actually it probably has a lot to do with the mechanics of the wave
itself
and the sea floor/bed around Diego Garcia itself. The wave may have
'flowed'
around Diego Garcia, but when it reached Somalia it had nowhere else to go
but
up the beach.

I watched a simulation that showed the main "wave" energy was directed
squarely
towards Sri Lanka. Probably because it was paralell to the fault line near
Indonesia.
While watching the simulation I noted that DG was not in line and quite a
bit outside
(flanked) of this main wave energy.

It's pretty simple physics.

Fluid dynamics is excruciatingly complicated.