The language in the link says: "They used to make orange filters to dim the primary RF image in screwmount Leicas, and this did improve contrast - but it darkened the primary view in the process."
Yes, Leica did make such filters for use on LTM camera -- called ORAKO or OKARO depending on which camera you'll use it on (picture below).
But as we know, there are two windows on an RF camera: the main window we view and focus in, the the second window that provides the rangefinding function. A filter would "darken the primary view" if you put it on the primary window. Put it on the secondary window, as I and as I expect most others do, and the primary view is untouched.
