That hook is always called. Even if the bomb planter left and even if the bomb item on the map is removed (in the latter case x and y will be 0).
It looks like the player ID will still contain the ID of the player who planted the bomb. Even if that player already left / a new player joined and received the same ID.
The same value is also used for the scoring system. Therefore this special edge case can probably even lead to a bug (wrong player getting extra score for bomb planting without planting it at all).
I guess it's a bug that the score is still affected when you return 1 in the hook. Can't tell you if this will be changed/fixed some day. Probably not.
Regarding range:
See info_bombspot - it's in int[0]. You can use entity to get that value.
The explosion damage value is 500 when you're exactly in the center of the explosion. The default range (when a range of 0 is specified) is 400.