This theory is somewhat plausible, however; with an inflight fire (having had one myself) I would find it very unlikely that one of the pilots wouldn't have taken two seconds to key the mike and declare an emergency. Also, emergency procedures for an active inflight fire is typically to land as soon as possible and Pulau Langkawi wasn't the "closest, safest airport." Kuala Terengganu was much closer, has an 11,000ft runway and has an approach over the water. Kota Bahru was also much closer with a 7,000ft runway and an overwater approach. As a matter of fact, the airliner's entire route from where it turned to either of these airports was overwater so why fly an additional 200NM and cross the entire peninsula, an area in which they had absolutely no chance to ditch if necessary?
Also, the writer forgets that reports on the radar return show MH720 fly over Pulau Langkawi to Pulau Perak where it turned NW, not to the airport.