This is true. However requires custom approach for other planetarium apps, such as SkyChart and HNSky. Moreover, none of this apps have the location configured before first start, so none of these panels will work before a user configures location in Kstars, SkyChart, HNSky etc.
Long lasting discussion about authoritative source of location in linux ended up in creating gpsd service, which is supposed to be system wide source of location based on GPS readings. It can be shared among applications by just using gpsd service. So the conclusion would be to use GPS or die

However, in our hobby there is lots of cases where you need to use precise location and don't have GPS. It is pretty much specific, because a regular user is just happy with location precision which can be established by geoIP or just timezone. So how can we handle it? And I mean authoritative location info other than GPS and gpsd service. The simplest way is a system wide configuration file used by every application. Optionally you can use gpsfake that mimics gpsd service. The latter keeps paradigm as it is - gpsd is the main source of location.
Actually in /var/www/gpsdpanel directory you can find gps_test.log NMEA file which you can use for testing gpspanel.py - just stop gpsd (sudo systemctl stop gpsd.socket) and run gpsfake -c /var/www/gpspanel/gps_test.log
So from this perspective the simplest way is to create virtual GPS source which feeds gpsd service with static location data stored in a file e.g. /etc/location
This way KStars and other apps using gpsd can use the same location data as GPS Panel, Astro Panel, you choose what... Is is perfect solution? I have no idea