I'm not the author, but I think it's related to the following point about failing fast. It's not a good situation to be pouring yourself into a business that just plods along, never really in danger of going bankrupt but also never in danger of great success. A clear failure would be preferable because it would be a lot easier to walk away, pivot, start over, etc.

Putting 40hrs/week into a stable, increasingly-profitable business is hardly a slow death, though.

Neither is putting in 40hrs/week at a stable job at BigCo

You missed the part about "increasingly profitable" which leverages each additional hour to generate more income (which no job will give you).

No growth could mean you're still at the flat part of exponential. Linear means it's definitely not exponential.

wait, this makes no sense... there is no flat part of the exponential... the derivative of the exponential is the exponential!

I should have said the flat part before an exponential. Like a hockey stick.

I think he is being pedantic, because that part is also exponential.

and really not that dissimilar to linear growth during that stage of the curve...

Sometimes startups are funded early based on track record and opportunity. If they instead launched and demonstrated lackluster (linear) growth before seeking funding, they could have a much harder time convincing investors of the exponential growth potential. If you're going for VC dollars, you really need to have a compelling story about how you will grow to be huge, and having some history of linear growth is a counter-argument to that.

Along with what others have posted, it may mean that you've chosen a business model that only scales linearly.