This is a nice mixture of “all bets are off” (referring to an agreement that is no longer valid) and “hands down” (easily or unquestionably). A big thanks to Aryelle Young for unintentionally blurting this one out!
This is a congruent conflation of “hands down” and “slam dunk”, the first an adverb and the second a noun, both meaning easily. The context reveals the reader meant to say slam dunk, as he was referring to an arbitration case with a former employer that he expected to win. A big thank you to Mike Kovacs who heard this one on NPR’s This American Life last week.