Flight Radar24 ADS data is still dependant on enthusiasts being in range with proper equipment
Others have mentioned that other aircraft prior to and after MH370 were tracked their entire flightpath between Malaysia and Vietnam. I don't believe there is a lack of coverage from the ADSB receivers in the area.
Also the data from FR24 pretty much exactly lines up with that published by MAS, and other sources such as the Vietnamese Navy.
MH370 Flight Incident | Malaysia Airlines
The last known position of MH370 before it disappeared off the radar was 065515 North (longitude) and 1033443 East (latitude).
MAS does not indicate what type of radar their data comes from.
It would see that all that remains to be seen is whether the aircraft crashed there (as the additional altitude data from FR24 seems to indicate) or whether that was where the electronics ceased working.