If that happens to lead to a billion-dollar company, awesome. But the odds of business tells us that's probably not overly likely, so we're making sure we'll be happy even if that doesn't happen.
(Who knew merely being satisfied with millions -- single, tens, or even hundreds -- would sound like a humble goal in this context.)
You've assumed that 37Signals refusal to staff up and take speculative VC rounds means they're steering the ship away from being "a billion dollar mammoth". But 37Signals is more successful than most of the companies (yes, most) that take VC rounds, and has enviable revenue growth.
I think you're seeing a business model that doesn't fit the YC get-rich-quick mold --- and that's what it is, read the essays --- and pushing it into a "small business" bucket. The real world doesn't bucket like you want it to.