To add to ynot's post, it's probably impossible for the local level organizations (hatchery, FN, and Metro Van) to go through DFO to implement a closure that quickly. Enforcement of a closure would also be challenging when anglers could honestly say they didn't know about because it's an unexpected situation. Only anglers that checked the regs in the last 48 hours would be aware. Sure, they could put up signs to educate anglers but the hatchery staff would be hard pressed to design, print, laminate, and then delegate someone to go out and post them at popular spots. All that costs extra money too. My guess is that putting it out there as a request rather than a closure is the easiest, fastest, most cost-effective way to get it done at the regional level. It's not perfect but emergencies never are 
this reasoning is 100% correct.
We could not get a closure within the short notice of time. The level of clearance required was too high, so an official request was the best we could do. It worked for the most part. There's always some who want to do what they want to do but most stayed away.
AND I did manage to print a couple of notices and posted them in the most accessible places. I didn't delegate that job, I did it myself!

The pulse of water from the dam coincided with collaboration from Squamish fish guardians opening the weirs. We actually did it twice, once on the night of oct 18th, and then the once more on the night of the 27th... 4CMS released for 6 hours... it may not seem like much but as a result we had over 1200 coho and 40 chinook enter the hatchery. Before that, from the period of oct 1-19th we had zero fish coming in. I'm taking that as a success.
I'm also very happy to say that Metro Van, Squamish Nation, and the hatchery are working more closely with each other then ever before; it's taken several years to build these relationships, and there's a lot more we will be working on together.
Currently the reservoir is filled up to the bottom of the drum gate and is spilling freely.