Sure, but the political reality is that his tax returns had to be released. Like it or not, that is standard procedure these days. A substantial portion of the voting public expects and demands it, and Simon had to be aware of that political reality before he began running for Governor. Frankly, the only reason I can imagine for him thinking that he wouldn't ultimately have to reveal that information is if he never really thought he'd win the primary.
I doubt much of the public has even tried to look at Gray Davis' tax returns. The oft-mentioned counterexample here is Sen. Daschle, but there are other, less famous politicians who refuse, too. Most people just do not care about other people's tax returns beyond the curiosity of other people's incomes.
the only reason I can imagine for him thinking that he wouldn't ultimately have to reveal that information
How about privacy for himself, for his business partners, and for his family?