It's more the case that that's not investors' biggest worry. Investors are willing to believe (usually correctly) that the kind of people we fund can build things. They worry more whether there's a market for what the startup is building, whether the founders can actually ship, how much money they can make, etc.
Much like the fact that my resume lists a $150,000 piece of paper means you can skip seeing if I can solve "3x + 4 = 19" at a hypothetical job interview, the YC label means that you can confidently say "If they were bozos at product they would not be here, so let's focus on the more important questions."
Is it a fact that the product execution by YC teams will be flawless, so we can just minimize it?