At the risk of sounding like a snob...don't follow the Tibbs-Vision instructions. There are much better[/url] resources [url=http://www.homebrewsake.com]available
on the internet.
With that out of the way, your question:
The temperature that you are referring to from White Labs is the optimum fermentation temperature for that yeast. That means that the yeast ferments the fastest in that temperature range.
As brewers, we're not necessarily interested in fast. We're interested in quality. In regard to sake, that means the closer you can keep your fermentation to the 45ºF-50ºF range, the better off your sake will be.
I hope that helps.