Product-market fit is not about that. It's not about whether you can find somebody who wants to buy your product. It's not even about whether you have very satisfied customers.
Product-market fit is about having a product that fills such a deep need in the market that you get pulled into an exponential growth trajectory. Instead of growing because of your own marketing efforts you grow because you can't keep the customers/users away. It can be a viral app where every user on average invites more than one other user. It can be an app that every college student wants to be on. It can be anything. But it's always about having some kind of self-reinforcing loop. It's about push versus pull. Peter Thiel referred to Twitter as a clown car that fell into a gold mine. Twitter had such strong PMF that all their operational mistakes just didn't matter.
Users make content. Content makes the platform better. Which results in more users. See step 1.
This is fundamentally different from doubling your signups after you double your adspend. Most businesses don't have product-market fit in this way. Most startups don't. That includes most unicorns. But those that do end up getting huge, and that's why startup investors talk about product-market fit and network effects and viral growth all the time.