[broiler]

I don't know about Phil Ivey, but Barry Greenstein, Matthew Hilger and Phil Hellmuth have all stated in interviews that I have seen/read that they file a Schedule C for their gambling income.

I think you missed the important sentence in the first paragraph in the decision:

In redetermining respondent's tax deficiency, the Tax Court held that he was in the "trade or business" of gambling, so that no part of his gambling losses was an item of tax preference subjecting him to a minimum tax for 1978. The Court of Appeals affirmed.

The Supreme Court upheld the decision, therefore gambling can be a trade or business.

Self employment taxes would be calculated on the net. In this case, the IRS was not trying to collect SE tax. The goal here was to hit the taxpayer with AMT because of the gambling deductions.