The yeast was in fact from decades ago. I believe Finn gave Charlie the yeast sometime in the mid-80's.
While filtering does certainly remove most of the yeast, it does not remove ALL of it. Mature yeast cells are typically 3-4 microns. Buds are much smaller. Some yeast cells do slip through the kieselgur filter bed.
How do you think White Labs and Wyeast obtained all the different strains they have?
The COORS R&D lab certainly had the tools and knowledge to obtain a single cell from a keg of Bud and to grow a culture from that.