This is good advice but on a practical level isn't clearly compatible with other advice like "fix the plane while it's flying".
Also I was under the impression YCombinator focuses on helping companies grow and organizes a demo day 3 months later to help them raise money from VCs. The pg article even mentions growth targets during YC. If this is true then companies accepted to YC should already have a product people love and be ready to hack growth over the next 3 months. This clearly is not the case (eg Airbnb joined YC after about a year not 1000 days).
If YC is not only about growth but also about helping companies at much earlier stage still working on the product people love stage, that would seem to be a challenge. What do YC speakers talk about? Growth or Product? It's like putting kids learning Algebra in the same class as those ready for Calculus, not easy at all. Maybe there should be two types of YC sessions then? One for early startups still working on product, another YC for startups working on growth...
Every founder I have ever met wants their product to be loved, but unfortunately they need time and money to figure this out. Other issues: it's hard to know when to walk away pre-product and pitching VCs without growth doesn't seem to work. So founders raise money by showing early "growth" without PML (product/market love), then they go back to work on the product and when PML doesn't materialize in the first couple months they start to get nervous and switch resources back to growth hacking their mediocre product so they can be ready to raise money again and have another chance.
Obviously this is all very complicated, full of exceptions to rules. In reality product and growth have to be going on at once - but the timelines and resources and challenges to do product and growth at the same time should be part of the advice.