The justification is contained at the end of the sentence you're quoting.

I disagree:

to at least give people a small idea of what is going on with him, so they can gauge risk

That's not in his best interest. It is in the stockholder's best interest, the company's best interest and (perhaps) that market's best interest.

I fail to see how it is in Jobs's best interest.

It in Job's interest to serve his shareholder's interest.

As Steve is a shareholder, if the stock price has uncertainty about his health factored in, and Steve announces his health is ok, the share price will likely rise and perform better going forward. If he is selling shares periodically to generate cash and diversify, he will get a better price.