Calm down people, he's being facetious. He's saying he's never filed taxes PERSONALLY.
But OF COURSE, every ORGANIZATION he has ever fought for (UFC, StrikeForce, EliteXC etc.) is obligated to deduct taxes / social security / medicare from his paycheck before he even sees it. He has no say on that. If the orgs aren't doing it for every salary they show on their payroll, they'd be shut down in a minute. In addition, it is unlikely that a guy as controversial as Diaz is getting many endorsements, so that pre-taxed fight org paycheck makes the most of his income right there.
So yes, he is TECHNICALLY telling the truth about never paying taxes himself, but in reality taxes have being going from his side whether he likes it or not.
Note that this is only on declared income such as fight purses and PPV cuts. This does not include locker room cash bonuses, whatever cash he makes on the side from coaching etc., but he would be far from alone in not "declaring" and paying tax on that cash income, if any ... that's the whole point of cash income, most people avoid paying taxes on it... there is essentially no way for the govt. to know about it, track it or tax it when it never enters the banking/credit system and is used for cash transactions.
No he isn't going to jail over this, and yes we can watch him get his ass beat sometime again in the near future.