Roselare Red additional Yeast

I have a Roeselare Red in the Secondary now which I plan to leave for 4 to 9 months (or more) to get the sour cherry finish that the style is suppose to represent. I received Lalvin EC-1118 Saccharomyces Bayanus Yeast that came with the William's kit. The instruction say to add this prior to bottling. I thought the Secondary fermentation was suppose to sour the beer, what does this additional yeast do? My guess is that the length of time in the secondary kills off sufficient amounts of yeast that the addition is required along with sugar to carbonate the beer. Am I close??

The low ph (sour) is hostile to sacc yeast. Chances are it will all be dead when you bottle.

The brett would eventually carb it, but it would take a while.