I think the only place you can get official confirmation that capital gains on US treasuries do not get an exemption on California taxes would be to read the California instructions and publications about capital gains. I'd look first at the instructions in the booklet, regarding adjustments made on form 540-CA, on the line about capital gains. Perhaps there are also some other publications that can be found on the franchise tax board website about this.
That being said, I've dealt with treasuries on my taxes for a long time and I'm pretty certain that the exemption you are imagining does not exist. The exemption is only for dividends/interest. I have never once anywhere seen any other exemption mentioned. Of course, I'm just some person on the internet. You could search around online to see if you find anyone discussing this. But that would also be just people on the interent saying things.
At the end of the day, reading the California instructions and booklet is really the best choice. It's is the official word on all such matters. And frankly, I often find the California (and IRS) booklets to be much clearer (and more detailed), as well as up to date, than information that Intuit provides or than what I see on these forums.
