It says on the site that it's usable as a LR plug-in too.
The "Luminar Libraries" feature provides Lightroom-like photo management, but based on Luminar 3 reviews it's probably not going to be as rich as Lightroom's. (I've pre-ordered Luminar 4 in hopes that it's good enough that I can ditch Lightroom).
There's a big vendor lock-in thing going on, isn't there? I've got 10s of thousands of images cataloged in LR. It would be hard to move away from that and re-index all the images again.
It also works as a plugin to Apple Photos, allowing you to use Photos to organize, and Luminar to edit them.