He cheats at everything else so why would anyone expect his golf claims to be true?
Golf is a curious sport with unique components not found in most other activities. One of the most striking elements is that, in general, there isn't a referee adjudicating every stroke. It's a game of honor where the players are required to enforce the rules on themselves. There are times when an official ruling must be determined in competitive play but that doesn't happen often and almost never in most recreational play.
There are two basic approaches to the game: keep a strict adherence to the rules and keep score mercilessly OR go out and have fun and don't fret over the rules or the scores. If a player keeps an official handicap with the USGA, it is understood that they will play strictly by the rules and post every score they shoot for calculating their official handicap index.
Since Mr. Trump cheats, (and in addition to Alice Cooper there are others who've observed Mr. Trump's casual approach to the rules) his claimed handicap is suspect. The false equivalency of Bill Clinton's liberal use of mulligans illustrates that Mr. Clinton plays for fun, that is, he doesn't claim that he's a single-digit player, (for non-golfers, that's an amazingly difficult achievement requiring hours of training every week). Here's an interesting article about Mr. Clinton's golf game:
http://www.golftoday.co.uk/news/yeartodate/news00/clinton.html
Conversely, Donald Trump makes outlandish claims about his abilities. In New York, there's a public course that's managed by Mr. Trump's golf company. It's ridiculously expensive but it's quite beautiful with panoramic views of the NYC skyline and the upper East River. On a par-3 hole, there's a big brass plaque alleging that in the inaugural round, Mr. Trump got a hole-in-one. It sounds to me like a claim Kim Jong-Il would make, and I just don't believe it. That's because Mr. Trump always lies and his arrogance knows no limits.
I cannot imagine spending four hours playing a round with Donald Trump. That would be hellacious. And I hate it when someone cheats and thinks I don't notice it. It's insulting because only one of two things can be true: the cheater thinks I'm too unobservant to see the cheating OR the cheater thinks I'm stupid.
It's the same with Mr. Trump's campaign. He thinks we'll not see his blatant lies and contradictions OR he thinks he's so much smarter than we are that he can gaslight us with his lies.
Donald Trump is an idiot.