It is quite a widespread belief that open source software is synonymous with malware. Brobably born out of the FOSS FUD from the late nineties.
Also, any association of Luminar with Lightroom is a good thing for Luminar in an advertising sense. It makes the two products seem equivalent, even though realistically they are not.
Conversely, association with FOSS makes Luminar seem equally “worthless”. That is probably not a media image they want to promote.
Note how none of this has anything to do with the quality of the software in question.