Maybe people here are talking about different things, or have different ideas of what strength is.
I do not believe that paleo man was bench pressing 300+ lbs haha.
I do not believe that being able to carry 100lbs on your shoulder for a few miles is STRONG. Its stronger than your average person, you're going to be tired at the end of the day, but to say that you're one strong mo-fo.. I think we just have different ideas here.
You can get good results from infrequest HIIT training, but to say that its the most effective way to build massive strength is a bit of a stretch.