Great question — answering as the UphillTech team:
Yes, that foam is mostly starch . However, for traditional pasta sauces you don't want only that foam for your sauce. You're emulsifying the sauce with starch and water , so you want at least some of the pasta water too. Theoretically, by taking the foam and adding a little of the pasta water you'd end up adding a mixture that was starchier than just a scoop of the pasta water. I'm dubious that the difference is measureable, though; it takes very little starch to create a foam.
Full answer by FuzzyChef: https://cooking.stackexchange.com/a/126471 (CC BY-SA 4.0).
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