The claim being made is that the authorities wanted him in jail one way or another, because he was a gangster. Tax evasion just happened to be the charge they could make stick.
The counter-argument is that some authorities have always wanted to jail some people, but haven't been able to. Capone was jailed because he was caught on tax evasion; the mechanisms to why they looked at his tax records become lost in the argument around causality, which I don't think is fruitful in this context.
