An example from the clip in the OP:
Participant has $10,000 in Match, is 60% vested and terminates in 2010, leaves money in plan. (assume no earnings)
At the end of 2015, $4,000 is forfeited per the 5 BIS rule. Account balance is $6,000.
Person is rehired in late 2016.
The $6,000 must be kept separate, as it's fully vested at the moment. Any new match is still subject to the vesting schedule, starting at 60% in 2016. After 2018, the accounts can be recombined (assuming participant accrued a YOS for vesting each year).
This has nothing to do with buyback of forfeitures, as others have said.