It's an essay relating his personal experience. (Notice that Seibel wrote: "Here is how I learned this about myself. [... rest of text ...]"
He didn't cofound a small-medium-lifestyle business so he wrote what he was familiar with: justin.tv/twitch.
Lastly, I didn't see any advice that didn't also apply to small-medium businesses. (See the 3 questions to ask oneself at at the end.) He's not advising anyone to start another billion dollar Youtube clone or a company that requires YC funding. One can take his questionnaire and apply it to starting a small bootstrapped 5-person consulting firm if that's what your idea of a "startup" means to you.
So this essay talks about startups under the YC brand, I assume we're talking about hi-growth business, not small and medium ones that have no goals to grow double digits every month.
Finally, we should stop adding a condescending "lifestyle" qualififier to businesses that are not trying to become a unicorn. It is most businesses actually.
It's also important to realize that a company's growth rate will vary at times throughout its lifetime. You might start out as completely self-funded, bootstrapping, and work very slowly for some period of time, only to reach a certain point and say "OK, now its time to turn on the afterburners and go seek VC / PE money". And as a company matures (hello, Microsoft, IBM, CA, HP, etc.) its pretty close to inevitable that the growth rate will slow down.
The important thing is just to do what makes sense for where you are and what your goals are.
"Lifestyle" is a neutral label and is not a condescending perjorative unless you insist on perceiving it that way. See:
When someone else calls a business "lifestyle" just because it is small, i believe it is condescending.
If I called the CEO of the 20-employees plumbing company i sometimes use, and asked him "how's your lifestyle business doing?" he would probably hang up on me.
Only problem: there is no way for YC to make money off of it...
So let's ignore S&M businesses in tech and as a valid entrepreneurship route. Let's pretend that the only options are a startup or corporate life.