"If strength training increased endurance then we should see elite marathon runners dominating in weight-lifting competitions as well."
Well, no. Marathon runners do not strength train. They should, but they don't.
As far as the individual that squats 1000 lbs, you must realize that he is also well over 300 pounds. His cardiovascular system would not be suitably trained and he would not make a good long distance runner (not to mention 300 plus pounds banging the knees and hip joints with each step). However, the weight lifter's leg muscles would have a higher level of endurance before fatigue than if the same individual could only squat 500 lbs.
You are mistaking endurance with metabolic conditioning. Strength takes years to develop because you are increasing the capacity of the muscles to perform work. When that capacity goes up, the endurance goes up. However, the cardiovascular system will not change. For that you have to train specifically for it. Interestingly, it only takes approximately 3 weeks of daily cardio training or approximately 20 minutes a day at your target heart rate to improve substantially.
However, increasing cardio has a less positive effect on endurance than specific strength training and it is not as long lasting. Meaning that if one stops metabolic conditioning, the positive effects drop off dramatically and quickly, whereas with strength training, the drop off is much more subtle and takes longer.
Nutshell: strength training takes longer to see positive results but the effects are longer lasting for both strength and endurance. Cardio training takes very little time to see positive results but the effects are comparably shorter lasting.
Just a quick comment about muscle fibres. Muscle fibres contract in an all or nothing spasm. You can't just tell your body to contract the slow twitch fibres or the fast twitch fibres. We are all genetically different and some are more predisposed to become a weightlifter and others are predisposed to become a marathon runner. What is key here is that strength training with repetitions between 3 and 12 (or so) have shown to increase the size of BOTH the slow and fast twitch fibres. However, specific endurance training with hundreds of repetitions have shown to increase only the slow twitch fibres. Now which type of training would be more efficient and more effective?