The way to understand torazu sanmoku is to recall that prisoners in black’s territory are removed from the board at the end of the game, before counting. Meaning, black does not have to close up his own territory with extra moves to capture the white stones. It is sufficient to prove that they can be captured in theory.
In this situation, counting black’s territory and prisoners, black has three points locally on the board as it stands. You can verify this by playing (locally) five black moves and also five white moves, starting with black.
It is important that both players play the same number of stones to keep the score balance of the initial position that we want to evaluate. Even if it means that white has to give a prisoner to black for balance (just put it anywhere in black’s territory).
Now, if black actually plays a move while the game is still on, he will end in gote doing so, and, having filled his own territory, his local score drops to two points.
The only difficult part of this is convincing yourself that the local situation is finished, played out as it stands, with black having surrounded three points, just with a slightly unusual border that is hard to visualize. I suggest that you try to play out the capture yourself, or you can look here for a more formal explanation. I find the wiki entry to be a bit too theoretical and hard to understand.
The three-points ruling is more fair and balanced than the 1989 Japanese rules, under which black has to play it out and lose a point. This is due to the quirky definition of seki, which is applied even though the position is not actually a seki of any kind.
Three points is also the score that you get under area scoring.
Edit: I take that back, I have no idea how to score this under area scoring. Probably 0 points.