Interestingly, this was a topic of much discussion among senior judges recently.
During the Pro Tour in Valencia, this past February, we did a straw poll of pro players; without exception, they felt that this should NOT be a “valid bluff”, but instead Adam has conceded, and we should hold him to that. And, to a large degree, the senior judges arrived at that same conclusion.
As to the argument (which Adam will certainly offer) that Nicole didn't show a Counterspell, well …
Counterspell is legal in Eternal formats, but neither Modern nor Standard, so
Cancel probably meets the criteria Adam stated.
Were this Legacy, and Nicole being tapped out with only Force of Will as a conceivable “out”, Adam could say “show me a Force and I'll concede”, and then ignore a Cancel (who plays Cancel in Legacy!??!?). This isn't that situation, however; although the original post doesn't specify that it actually happened at the PTQ, the discussion is still framed in that context - and PTQs are definitely not Legacy or Vintage…
d:^D