As you increase flaps from zero, the L/D ratio decreases; i.e. you fly a steeper approach or you need more power to maintain the glideslope.
When there's no more power available and you're coming up short, you need to increase L/D.
Decreasing flaps does that, but you are also increasing stall speed; so, in a low energy situation with very few seconds to weigh alternatives, not decreasing flaps further once a survivable impact point has been achieved sure looks like the best way to minimise the risk of something else going wrong.
Mmmayday38 had to negotiate a tight squeeze between landing too short and increasing the stall speed too much; he looked out the window and did exactly what was needed
