First we don't know where we'll be with vaccinations by mid-April, so we won't know the impact it'll have on future case numbers. If current trends continue California could be well over 50% vaccinated for ages 18+ by that point on the way to herd immunity.
Second while yes its possible that case numbers may go back up that doesn't mean that it'll be worse than last year. Once again the whole point of the vaccine is to lessen the severity of the symptoms and prevent those infected from having to be hospitalized and potentially die. So if a large portion of the population is already vaccinated by mid-April than case numbers won't make a dramatic rise. And will likely be a shorter time frame before they go back down.
Third I wouldn't expect theme parks in California will open before May anyways.
To be fair, the article did mention that the impending spike won't be as severe, nor would I expect parks to reopen before May, either. On the other hand, we still don't really have an idea for when things could reopen. It's not good enough for cases to go down, they have to stay down, and regardless of how severe the spike is, that may not be the case, which could lengthen the closure yet longer.