Military aircraft operating between Diego Garcia and Senkaku (or other islands in the South China Sea) would probably transit exactly through the area where the unidentified radar contact was plotted. Especially if Vietnamese an Indonesian airspace is to be avoided.
Air to air refueling might also happen in this region. Some military a/c is designed to have very small radar cross sections, so it might not be easy to identify plots of those a/c. The radar plots may well be totally unrelated to MH370. Support by FAA/NTSB makes sense.