I think he believed that he would die anyway or when considering his mortality, it may simply be a moment of bewilderment.

His choices were;

a) don’t flash, die. With flash remaining, Oner might focus other lanes, giving 369 a bit of breathing room.

b) flash, expect Zeus to follow flash, and hope Zues dies to turret. Now flashless, Oner will camp top lane.

I haven’t watched the video but assuming it’s the play I’m thinking of, he was expecting Zeus to use Q3 and was waiting to flash the Q3 but instead Zeus just followed with an auto and caught him off guard.