Aside from the fact that I expect that the TrueOS developers (and Ken in particular) are quite invested in Lumina from having spent as much time on it as they have, Lumina is something that the TrueOS developers have control over and can do what they want with. They don’t have to worry about whether KDE or any other Linux-centric DE works well on FreeBSD or whether it will continue to do so. They don’t have to worry about whether any upstream changes to the DE fit with what they want. They also don’t have the problem of the 3rd party DE working with FreeBSD 11 but not 12 (and thus not TrueOS), since that sort of problem does happen with some software.
Aside from the amount of time and effort required to work on Lumina, having Lumina is mostly all upsides for the developers, and they’re obviously able to spend the time to make it work, since they’re already doing it.
As a longtime KDE user, I very much want KDE to be a first class citizen on FreeBSD, and I’d much prefer to use it than have to switch to Lumina, but I also don’t see any reason for the Lumina guys to stop what they’re doing, and I wouldn’t expect them to, even if we somehow reached the point that KDE was just as well supported on FreeBSD as Linux.