Your examples are only true because gestation and many software projects are not easily parallelizable. It's a rule of thumb that applies to many scenarios, but obviously not all. If it takes 2 hours for one person to mow his lawn, then 2 people could mow it in about 1 hour. 100 people really can lift 100 times the weight of one person. Not to mention that nine women can have nine babies in nine months.

Making many different aircraft prototypes simultaneously is certainly parallelizable, although it obviously would be enormously expensive.