I'm also confused whether that quote is referring to the system inlet (where I thought spin was desirable) or fan inlet (and how would you control that?).
And in context is that quote about dust collectors where you are trying to separate material from air or ventilation?
I'll give you my opinion. Cincinnati is talking about the negative effect of air entering the inlet of a centrifigal fan with a radial spin. Spin in either direction effects the efficiency of the fan's impeller. I do not think it matters what application you are talking about, dust or ventilation, there is still an adverse effect. I would even go so far as to speculate that radial spin of a fluid would adversely effect the performance of a centrifigal pump.
When I was building and testing my Thien separator, I initially had it connected to my blower with about 6 ft of flex hose. I did not take any FPM/CFM measurements in that configuration because I wanted to eliminate the flex hose since it causes 3X the line loss of smooth pipe. However, when I close-coupled the separator to the inlet port of the DC (about 8"), the DC became much louder and when I put my hand on the housing, I could feel the increased turbulance. That is when I added an air straightener in the hopes of getting rid of the objectionable noise level. What I discovered after taking measurements with an anemometer, not only did the noise decrease, but the CFM's increased when the air straightener was added.
So, my experience suggests several things. The Thien separator imparts a radial spin on the air exiting the outlet pipe. That spin when close-coupled to a fan inlet will cause turbulence inside the fan that increases noise levels and decreases the performance of the fan. Since my system is a counter flow arrangement, my comments only address that configuration. However, I have no reason to dispute Cincinnati's comments that radial flow in either direction has a negative effect on on the fan's performance.