I think that's a pretty good explanation:
1. First whistle is always to stop play as the ref was told the time has elapsed, then usually the second whistle will indicate game over.
2. The captain's challenge was done after the first whistle and the second whistle was never blown so it is considered a 'stoppage' but game isn't over until the ref blows the second whistle.
As for the call itself, when I saw it in normal speed, I thought it was a 50/50. The more replays suggested it may have been a 40-60. Definitely not enough to overturn the decision.
No issues with the process IMO, definitely a bit of gamesmanship and smartness by Townsend/Feldt.