You seem to have the wrong expectations. No, it will never be as thickening as a cornstarch slurry. If that's the level of thickening you expect, you are really better off using the slurry.
Don't forget that pasta water thickening is a traditional technique from the time when people did not go to the supermarket to buy a pack of cornstarch. They cooked down ripe tomatoes for several hours, and the starchy water saved from needing a few more hours of evaporation. Also, they cooked with homemade pasta, which had some flour residue sticking to it, not the perfectly-gelatinized industrial pastas we buy today.
If this is not how you cook, and if you prefer pudding-thick sauces, then the slurry is probably the better method for you.
I notice Kenji from Serious Eats has also tested pasta water and recommends it for flavor reasons. He also tested it for thickening - but against salted water, not against a slurry. That's what people mean by "it thickens" - it thickens when compared to random liquids, not when compared to thickeners.