Take a deep breath and relax. This whole argument is simply because of the way the competition has been named.
Putts are counted as putts no matter what club is used after the ball rests on the green. Thus, a putting challenge only indicates that all strokes are to be taken after the ball rests on the green.
To limit the competition to putters only, WGT should be naming the competition using the term "putters only" instead of "putting". "Putters Only Blitz" is quite different from "Putting Blitz". That would restrict the club use as opposed to the current reference to location but not the club..
Whether or not we don't see eye to eye in these threads, it's not worth getting your blood pressure up arguing about it. Current rules allow it. Limiting a competition to "putters only" probably belongs in the "Product Suggestions" part of the forum.