Microbes can exist in fuel and can do lots of damage to the fuel. It wouldn't be a stretch for that to happen in oil if you designed the oil or the microbes right.Their response makes no sense whatsoever. There is no biodegradation going on in fuel. You need, you know.....a biome for that to occur. It also shouldn't be biodegrading when exposed to air, unless you have a huge yeast culture floating around just waiting to colonize your fuel container. That 21 day time period should be based off of soil microbes though, which ideally are not in your fuel container.
Now the oil may have a mixed shelf life of 21 days, which would be a pretty poor shelf life, but it is not biodegrading within 21 days mixed in fuel.
Stihl has had biodegradable oil for a while. I think that the super they used to make was biodegradable.