It would be unprecedented, as far as I know, for a company to be liable for taxes on the employee's on-paper-only gains, occurring some arbitrary time later at option-exercise.
And yet: that seems to be what Section 902.1(b) says.
The company is on the hook for payroll taxes on the employee's share appreciation, at the moment of option-exercise. Even if the shares are not even sold for any recognized capital gain. Even if the person is no longer a company employee or resident of San Francisco. Even though the company may have already paid taxes on the then-current grant-value in an earlier year.
I not a tax lawyer but on the surface, this seems insane.