however if the propeller speed is reduced, it should be done after the throttles have been retarded.
Wow, that's awkwardly worded! As all Piper Seneca engines are direct drive, the propeller speed
is the engine speed. As you retard the throttles, and the propellers reach the fine pitch stops, they [engines] will slow in relation to the throttle position.
Yes, you can further reduce RPM by selecting a more coarse propeller pitch, though I cannot think of any reason to do this in a twin, other than to stretch a glide after a dual engine failure. Having the props selected to coarse with the power is very low is generally harmless, unless you suddenly advance the throttles (like to go around). If you advance the throttles with the prop levers well back, it will be very quiet still, and rapidly become very expensive.
I can't imagine what Piper envisioned when they wrote that, but I would avoid doing it without a profound understanding of engine operation, and a compelling reason. An engine operating at maximum RPM is noisy, but it is protecting itself from detonation. Reducing RPM while maintaining the power setting is taking you toward detonation conditions, and you will only find out afterward that you were there, with a maintenance bill or engine failure.