CodCart: To actually explain why this isn't seki: if white tries to attack, they self atari - obviously. But if black attacks (after surrounding outer liberties) then white will capture those four stones, however this leads to the 'bent four in the corner' shape, which is somewhat unintuitively a killing one.
Black plays at B19, and white must play at A19 to keep black from reducing eye space further. Black captures at A18, and white NEEDS a viable ko threat to keep black from filling in (and thus killing) at A19.
Because white can only live with a viable ko threat, and white cannot start this exchange without putting the group in atari, black can simply wait until white has no more ko threats (such as in the endgame) before finally making the bent-four shape, ensuring white can't do anything to save it. Hence the shape is considered killing, despite looking like seki at first.
It is called "bent-four in the corner" and it is dead, not seki.