That's not really how that works. First off, NXT is not the brand, it's a graphic that's a stylized shortening of the word "next". Second, whatever trademark WWE has for "NXT" would be limited to wrestling. In fact, since I found it, the registrations WWE has for "NXT" are essentially limited to merchandise related to the league and wrestling broadcasts on TV and online.
A quick search of the US Trademark database lists 200+ trademarks using "NXT" (including at least six that are exclusively the letters "NXT") as trademarks for things like
Artificial turf
Sports drinks enhanced with minerals; Sports drinks, namely, performance drinks; Sports drinks, namely, recovery drinks
Retail store services featuring manufactured homes
management of multi-tenant living facilities
Vacuum cleaners and vacuum cleaner parts
Semiconductor lithographic machines, namely, lithographic machines used to manufacture semiconductors, and structural parts therefor
Water treatment equipment, namely, water filtration units.
Electric battery fast chargers
TL;DR There's nothing stopping them from using a graphic that says "NXT" in promos.