Even "on to something" is a click-bait title which HN guidelines say should be neutered. IMHO you don't need to force it, just be classy and straight-forward e.g. "YC Startup School Radio Ep. 3 - Optimizely & Lawn Love" or something.
I know companies that did the same thing, only they were not onto something big. Having customers pre-product is a good signal. I'm sure that the class of companies for which this happens have a much higher success rates than a typical startup. However, that's far from knowing you've hit something big. That's something you can only say in hindsight.
Because of the .ly TLD. Many companies employed a play on words to have a single-word domain for any word ending in -ly. Bit.ly is probably the most well-known.
The problem is that .ly is managed by Libya, and they tend to have restrictive laws and government practices that many people take issue with. So, many companies just got the -ly.com versions of their domains and made the switch.
To expand upon your comment.... those startups used something.ly domains when they were small and just starting out -- i.e. before they could afford to buy something-ly.com. Once they had traction and raised money, they purchased the proper domain. It's the same reason TheFacebook later dropped "The".
It's good to work your network to get first customers but every startup can do that. It's just common sense.
And nowhere in the article Optimizely claims this was how they knew they were onto "something big". So the title is misleading.