I would think the ingredients could be processed in parallel. For example, the Chocolate Chip Cookies recipes quotes a total of 9 years 29 weeks to make everything, but really the longest lead item is the vanilla beans @ 4 years. So really it's a 4 year process isn't it?
Also, as a sidenote - each recipe actually has its own "parallel" and "serial" time. The parallel time is independent of quantity, and the serial time is dependent on quantity. For example, growing vanilla beans has a parallel time of four years (each plant will grow simultaneously) and then they have a serial time of ~1/2 hr per plant to harvest.