Why when measuring turbidity do we use the minimum wavelength?
Usually when measuring materials in a spectrophotometer we want to use the absorption maxima to minimize the signal-to-noise ratio, that's why for pigment measurement you'll use the maximum point. But when you measure two materials you want to take two measurements with as little correlation as possible, that is to avoid one material influencing  your measurements of the other.
Imagine you are trying to breed C. marina for more pigment. Obviously more algae = more pigment so you want to measure how much algae you have and normalize by it. If you'd measure alga density in a wavelength correlated with chlorophyll content you'll get inaccurate results.
I think the question is ill-worded. You should measure at the pigment's absorption minima and not at the alga absorption minima. As the alga can have other peaks unrelated to chlorophyll. When doing this you should remember that chlorophyll and many other pigments are insoluble in water but your measurements of live cells are, usually, in water media.
The best in my opinion and that is the method I'm currently using is to build your own curve by counting cells in some manner (like hemacytometer) and measuring the full spectrum (Your graph stops at 750nm, I got up to 1000nm).