The Great Sumatra fault is right-lateral strike-slip, and would not trigger a large tsunami. It could trigger a local tsunami is there was a large submarine landslide let loose by the earthquake. The other future quake they were talking about is a subduction quake. It could cause a tsunami....but to cause one comparable to this one it might have to rupture a much larger patch than failed in 2 quakes there in the 19th century (it would have to rupture them together...although one alone could cause a significant tsunami. Other parts of that subduction zone face more to the south and could cause problems for other parts of Indonesia....
(I'm not an expert on tsunamis...)
Chris