While growth is the most important thing in a startup, it's unreasonable to expect it at the point when companies are applying to YC, so it is not the main thing the partners look for. Neither Dropbox nor Airbnb nor Stripe nor Optimizely had any growth when they applied to YC. When I was reading applications (odd yet delightful to use the past tense) the main thing I looked for was energetic founders working on an idea that grew organically out of their own experience. E.g. Optimizely consisted of the guy in charge of a/b testing for Obama's first presidential campaign, starting a company to turn the insights he'd gained into a product.

That said, startups should certainly seek growth, because that's how you evaluate and refine an idea. When you try to get people to use something you've built, and especially to pay for it, you learn quickly what's wrong with it.

The only misleading thing about Adrian's advice is the implication that you need growth to get funded by YC. That is definitely not true. A quick random sample of the current batch (the first 10 in alphabetical order) shows only half had any growth when they were accepted.