I think you’re confusing a startup with a startup unicorn. You can start tomorrow a niche SaaS, e.g. a debt collection CRM for French market, and become a startup.

Are you successful startup? Yes (e.g. you turnover $20k a month solo). Are you disruptive? No.

Truly disruptive startups are in two digits range. Successful startups that “made it”, whether it’s revenue, money raised, or profit, are in the hundreds of thousands/millions.

According to YC (at least Paul Graham's classic definition) a startup is something that grows very fast.

It might be a startup in the colloquial sense of the word, to friends, family, etc., but I and many others here would argue that it's a small business rather than a startup.