I believe that the game should (have been) be a draw, not a win for black. One task that black must take on in this situation, where they are in such control, is to manage the situation so that they do win. If they cannot do this they do not deserve to win and playing for the draw (successfully so in this situation) should be an option for white. The rules should definitely NOT be changed mid (or post) game.

I have just checked the rules and see that there is now a 3rd win condition that I do not remember before which references the "no valid move" situation. I expect this has just been added. This scenario was previuosly discussed in the Tablut DB without any change being considered appropriate then so I don't see why one is pertinent now. I do not agree with the rule change and believe that the game (would be) is devalued with this change ... just my opinion though.

*Edit* I have just done a search of other internet sites with Tablut and other similar variants. While some rules are slightly different (number of pieces, size of board, need for King to reach the corner, capturing against the King as well as pawns) none mention a rule whereby a win is awarded against a player not able to make a valid move ... I STRONGLY believe the rule change must go.