The SMPC does make it more difficult for votecoin-holders to coordinate, but more importantly, it makes it impossible to bribe them.
Or is the argument that the votecoin holders are unknown and therefore no one can "bribe" them?
No that is not the argument. The list of votecoin holders is publicly known.
what about an additional contract that will pay a bounty to all votecoin holders if a specific result is reached?
Even if such a contract is made, the optimal strategy for a votecoin holder does not change. An incentives chart:
oracle outcome | truth | lie
you lie | lose votecoins | get judging fees and gain bribe
you true | get judging fees | lose votecoins and gain bribe
Each voter has control up-down, and no control left-right.
Consider what it looks like if a votecoin holder is gambling on his own bet:
oracle outcome | truth | lie
you lie | lose votecoins and lose bet | get judging fees and win bet
you true | get judging fees and lose bet | lose votecoins and win bet
Notice that it is the same thing as the bribe again.
If the market is popular enough, then votecoin-holders can create insurance for themselves this way to insure themselves against the randomness of bribes, resulting in incentives like this:
oracle outcome | truth | lie
you lie | lose votecoins and win 1/2 bribe | get judging fees and win 1/2 bribe
you true | get judging fees and win 1/2 bribe | lose votecoins and win 1/2 bribe