The point that arearadar was alluding to (and clarified in a PM) is that the original air defence track may well have been correlated with the civair SSR flight.
At the time SSR was lost and the only tracking was from primary return the military track was probably of MH370.
When radar contact was lost the probability confidence has to drop.
When radar contact was apparently regained the probability that the regained track was MH370 was depended on fewer values.
1. No other unidentified air track was known to fly that course.
2. No other primary radar track, uncorrelated with SSR was know in he area at that time.
etc etc.
In otherwords, the certainty provided by track conformity, SSR, communications, continuous primary tracking, was lost.
It was probably MH370 but it was not certainly MH370.