Yes, that's the definition of unicorn. There's companies like Avast on here because they stayed private for a long time before hitting the gas and getting to that.
Because of the way the equity markets have been recently, there's not a clear path to becoming a unicorn without hitting most of the other markets we tend to think of as startup-y: VC-backed, high growth, scalable into a big market.
If that is the case, there are a few dozen "unicorns" that never had a "hit the gas" period and just grew slowly into billion-dollar companies over decades. I don't see any reason that is not a clear and viable path today, if you have the patience.
I guess in a world where jet.com and Slack exist, it's easy to see many of these as "slow" growth. Avast has been 56% annualized growth over 2008-2014. Getting these valuations typically takes high $X0mil revenue or more, meaning most of them over a 7 year period of annualizing their growth at 50%+. 50%+ growth is pretty impressive, and I don't think there are many people who would think of that as "slow" or "patient".
Because of the way the equity markets have been recently, there's not a clear path to becoming a unicorn without hitting most of the other markets we tend to think of as startup-y: VC-backed, high growth, scalable into a big market.