If anchored putters are so great surely everyone would use it. Since not every one uses one then I don't see how it is a proven advantage so I say he is not cheating.
How about asterisks next to anyone who uses metal drivers, these are now universally used so there must be a benefit that is proven otherwise no one would use them!!! just kidding!
If long putter are easy to use then why wouldn't the big manufacturers force their players to convert and win and then reap benefits from increased sales? Imagine if Mickleson broke his major drought with a long putter and then said that it really helped, every second golfer would have switched. Woods has not won a major for a long time, imagine if he now switched and won, I bet Nike would sell a ton of long putter tomorrow.