Opponent played "Brilliancy" and lost 2 moves later

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TheSonics


I made a stupid blunder (a completely not working sacrifice)

To which my opponent replied with a "brilliant!" calling of my bluff...

Then he blundered M2... 

Of course in hindsight its easy to spot various ways he can stop mate, maybe engine considers it hard? ... but is that why he was awarded "brilliant"? 

How does "brilliant" work? I don't get it  

...17. Qxc6!!  

https://www.chess.com/analysis/game/live/23663494349?tab=analysis

TheSonics