The LNP itself is supported by the fact that it lies approximately 240 miles from the radar antenna on Pulu Penang: exactly where we would expect the horizon to block a conventional radar signal. So we have verifiable data to work from.
MH370 went off the radar on Penang Island at a range of 200 nm. If that was due to the Earth's curvature, the plane would have had to be at an altitude of around 12,000 feet. That conflicts with the reported altitudes, so I suspect contact was lost for other reasons than the horizon blocking it.