Good morning Pete!
The decision to allow a profesional POKER player to be in a "trade or business" is:
Baxter v. United States, KTC 1986-57 (D.Nev 1986)

It is a United States District Court case found in favor of my friend Billy Baxter. Billy is the GREATEST low ball ball poker player in the world. In addition, he managed three World Champion prize fighters at the same time.

The decision is the basis for my tax treaty book I publish every year, and have for the last 10 years, involving non resident aliens (foreign poker players) who win at poker tournaments. The issue being whether the casino should withhold 30% from the individual poker tournament payout or win for federal income taxes. Each foreign country is different and requires analysis.

There is no legal basis or precident to allow any other type of professional gambler the support of being in a "trade or business". Poker players are the ONLY exception because of the Baxter case. In addition, any poker player claiming a status of being in a "trade or business" better retain a quality CPA to defend the issue if it should be audited.

The article, in my opinion, is very misleading because it creates an illusion that ANY gambler, regardless of the game played, is entitled to be in a "trade or business" of gambling. It just isn't so.

As far as "their" book goes - each to their own!
Thanks for asking,
Jim