Have I missed it somewhere? How do they Know :
A. The radar tracks WERE 370 without IFF
B. The origin of the "waypoints"
Does MAS subscribe to ADS-C?
Please do not de-bunk unless you KNOW and can give the evidence!
To the first point, in addition to what Pontius Navigator posted earlier, the authorities will have the data from the Inmarsat "pings". Since it has been reported the plane they believe to be MH370 disappeared from military radar in the 2:15-2:40 time period, they can use the distance to MH370 at 2:11 based on the Inmarsat data. If the arc proscribed by this distance matches up with the radar returns at the same time, that will be additional information to support their conclusion.
Additionally, you will have significantly different Inmarsat distance data if the plane did a 180 and tried to return along its original heading as opposed to making a left turn to cross over the peninsula along the Malaysian/Thai border as described by the authorities.
I believe they have a very high degree of confidence that the plane they saw was MH370.