Basically, she said on her tax return that she had exercised her options on April 4, 2001, when not only had she not done that, but she hadn't yet met the requirements for being able to exercise her options at that time. This is not being burned by some esoteric rule where she checked Box A when she should've checked Box B. She filed a tax return that said something happened on a certain date that not only didn't happen on that date, but couldn't happen on that date.
That's why Horowitz's premise is false. He wouldn't have gone to jail for implementing the same backdating scheme. Tons of companies did it, very few went to jail, and those who did went to jail because they let the backdating fiction cause them to either lie on their tax returns or commit affirmative fraud on investors.
Also: to make a more general point--companies are entitled to compensate executives in whatever manner the shareholders will tolerate, but public companies aren't entitled to be deceptive about it. That was the problem with backdating: while the process itself was legal from an accounting standpoint, the fact that it was built on a fiction made it easy to cross the line into outright deception. The wikipedia article actually has a great sentence that captures the whole situation: http://en.wikipedia.org/wiki/Options_backdating ("To be legal, backdating must be clearly communicated to the company shareholders, properly reflected in earnings, and properly reflected in tax calculations.")