I can't downvote posts yet, but if you have the ability, consider it. This is clearly LLM slop without human review.
I really hope you're talking mostly about header files; for actual code this is such a horrible idea (and in a lot of cases just won't work without massive efforts.) Even for header files, arguably one ought to really know what they're doing.
The article never even discusses how C++ actually implements unions to begin with, so it's just incomplete. (Only the "active" member is meaningful, the others are considered as meaningless and shouldn't be touched.)