Your buddy's GREAT soup is a perfect example. I read that a single win in cooking can create a lifelong habit. Now that trick is probably his GO-TO move.
Honestly, pasta water often makes soup too starchy and weak. The leftover water lacks real flavor compared to broth or stock. It's a cheap move, but not one that leads to great soup.
When lunaf16 says pasta water is too starchy... that's actually what can make soup better. The starch helps thicken it up so it feels richer without adding anything extra. It's not just a cheap trick, it's about not wasting good stuff that's already flavored. My family always saved pasta water for soups and sauces... it gives them a smoother texture. If you salt the pasta water well, it brings a subtle base taste that blends in. So it's more than just weak water, it's a simple way to build flavor from what you already have.