https://www.figma.com/blog/design-meet-the-internet/
They started building in 2011/2012 and then had their first limited preview release in Dec 2015.
How many other companies today are building for years before their first preview launch?
Especially as prevailing wisdom has turned to launch quickly and iterate instead of crafting, iterating, and refining a product first.
I'd also say that Figma is a high complexity product that would take a lot of time to get right. It's not impossible a small team could beat the odds, but it's not something that a hacker can knock out in six months with that level of refinement.