They likely had enough fuel to reach Beijing with a bit extra for emergencies just to save costs. Let’s say that it was enough go-go juice for 3,000nm. If they were flying with a full fuel load, then they would have a helluva range.
For the record: Kuala Lumpur is roughly 3900 miles(3300 nautical miles) to Mogadishu.
Seems a bit far for a passenger/hijacker to be confident there would be enough fuel on board in order to make it.