Hey Mike,
thanks for that!
I guess they added that aomewhen in the last years… 
Anyway, in the help it says something else about the preselector:
RealityCapture Help
“This is the number of features that will be used in alignment from the detected ones. Optimally, set it to 1/4-1/2 of the detected features.”
The problem with this statement is that I regularly get images with more tie points (not features!) than the preselector. That should not be possible according to the help…