Learn English – meaning of Trump’s “stable genius”

I think what is going on in the Urban Dictionary definition you found is a description of someone at a carnival who is purposefully using reverse psychology in an attempt to actually get more people inside. I'm getting this from the definition's example sentence (Carnival Barker- "Don't go in there man it's haunted.") For example, such a carnival barker may target a man and a woman on a date. By saying to the man, "don't go in there, it's too scary," he could be seen as indirectly questioning the guy's "manhood," thus convincing him to go in and prove himself.

This is an admittedly confusing definition of carnival barker. For future reference, Urban Dictionary is often useful for very current slang and fun to play around with (try looking up your town or city), but it's not the best place for good definitions. Check out these "definitions" of Urban Dictionary at Urban Dictionary to get my point. And even for definitions of slang, be sure to check the poster's other entries for a sense of their credibility and take note of how many up vs. down votes their entry has.

As you indicated in your question, double doun originated from the card game. It can also mean:

To double or significantly increase a risk, investment, or other commitment

When politicians make a comment on someone or something in a seemingly wrong or politically incorrect way, they either back off by making some excuses or double down on (stick to) what they said by restating the reason why they said so.

Trump suggested that George W. Bush bears some responsibility for the 9/11 terror attacks on America and he is being criticized for the remark.

Triple down is a more exaggerated expression than double down as it more significantly increases the risk of doing so.

The author could have used "double down" in place of "tripple down", but he/she wants to emphasize it is not the first time that Trump doubled down on an issue. Next time Trump implicates someone like Bush in anything, the author might use "quadruple down".