I think the bigger danger is when it's in the charging case. They're more likely to be tossed in the bottom of a bag or pocket where there will be no ventilation whatsoever. On their own, the airbuds have a much higher ratio of surface area to battery volume, increasing cooling especially when they're in-ear.

Still doesn't seem like a big deal because Apple usually demands the best binned parts for their products and they have weight to throw around. Better quality battery cells are far less likely to have fires due to thermal runaway and Apple power firmware is probably good enough to stop them from reaching that point when charging.