At Delft the sun was already below the horizon at ground level. However, an already set sun lights higher altitude clouds and air to give a twilight glow.
But this intense light was very likely something different - A "Zero Order Glow". It's an 'unofficial' term, I coined it more than a decade ago and it seems to have stuck!
Rain was likely falling in the west. When sunlight falls on a raindrop, most of it emerges from the other side. The curved surfaces refract the light and the leaving rays splay out over a wide angle. These rays - highly reddened as the sun is shining through much atmosphere - lit up the western sky with an intense diffuse glow.
A small fraction of rays entering a drop reflect instead off the far side and emerge towards the sun. These rays form the primary or 1st order rainbow.
An even smaller fraction reflect twice to form the 2nd order or secondary rainbow.
Why "Zero order glow" and not "Zero order rainbow"? The zero order rays spread through 180° without concentrating in any direction. They do not gather or cross to form a rainbow. We see a diffuse glow in the sky and not an arc.
When you see a rainbow and the sun is low, look sunwards for the glow!