This is not about a dim viewfinder, but for a faint rangefinder spot in the viewfinder of rangefinder cameras. It works by increasing the visibility of rangefinder patch by eliminating the direct image on that spot of the viewfinder. It does not make your viewfinder brighter, cleaner, or improve it in any way.
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.