+x+x+xYou're not doing anything wrong. That field filters the airfields displayed when you open Direct To in flight. It's not designed to remove airfields from the map.
Removing airfields from the map just because they have shorter runways is not a wise thing to do.
I land at 80 knots...
You may not be able to land there, but knowing that there is an airfield nearby with possibly associated traffic is something I consider valuable information.
Also, in the event of an engine failure, in many circumstances it will be better to land on a prepared strip that is nominally too short than take your chances with a much worse surface.