The 5/31 date was referring to WA, not CA. However, for CA, the Clean Vehicle Rebate Project has certain amount of funding each year and the rebates will be paid out through the available funding. In the past, the funding has run out mid-year.
If funds are not available at the time of your application, you may still apply and be placed on a rebate waitlist. Rebates for approved applications on the waitlist will be issued if additional funding from the state of California becomes available. If funds are available at the time you submitted your application, your rebate amount will be reserved when you apply online.
While there is always the chance that additional funding will not be approved, the likelyhood is that it will be. If the funding runs out, you just end up waiting longer for the rebate.
Clean Vehicle Rebate Project