It’s within range, assuming the plane had the minimum amount of fuel. But it’s a tiny island — filled with and surrounded by radar and communications — and there’s no way a plane could land there undetected. If you want to believe the US hijacked the plane, go ahead, but please remember to put on your tinfoil hat...
Diego Garcia is a base for (among other things) big ugly flying (errr) fellows ;)
In other words .... not a place where anyone can land with impunity.
;)