Yes, rainbows like bosemani are freshwater and ideally should be kept that way, but they won't suffer any damage when kept in brackish as long as you keep the rest of the water parameters ideal, in the same sense as mollies and brackish BBGs in freshwater.
Seriously,I wouldn't trust anyone religiously. I don't think anyone considers those rainbows brackish, just as no one would consider a black molly to be freshwater. But they can be kept in brackish and still thrive.