The devil's in the details. The handbook emphasizes Growth Growth Growth. So when your small focused obsessively cohesive product saturates its small niche and your growth is suddenly not-so-exponential, if you're a startup you start to Expand Expand Expand your product, new use cases, new functionalities, until you've reinvented email again.
If you're less growth focused, have your pricing nailed down, and are well positioned in your niche - then you should have enough new users from simple population turnover and you don't need that graph to keep angling so sharply upward to avoid ulcers.
The difference, I think, is that a lifestyle business is a startup where you take your foot off the gas once your income meets your personal needs. It doesn't need to be a monopoly - in fact, it's better if it isn't, because if it is some other company will likely monopolize it. And you don't need to deal with the fundraising and hiring parts of the handbook, since you never get that big. Everything else seems to apply, though.