That strain is pure saccharomyces, nothing else but the one saison yeast. Unless they sent out an infected yeast packet, which is very unlikely. I'm not sure what you think is strange? The beer could have been infected during primary fermentation or before it even started, but how would you have known? The pellicle is just the yeast/bug/bacteria/whatever using the oxygen on the surface of the beer, and could not form during primary fermentation. They are definitely throughout the beer, not isolated on the surface. They just like the oxygen and so they grow/thrive on the surface in a visible way.