He has a point, but anyone who has ever built something non-trivial from start to finish already knows this. It's the subtleties, not the broad concept, that drains time.
That's why the "weekend" meme persists. It's not about being "done" (software is never "done" anyway), it's about getting it to a point where "done" has meaning.
The little stuff can always be done. But my fear is that the people who insist on mocking the weekend hackers are the people who refuse to start on the big thing for fear that they'll take too long.
Regarding a HN clone:
Sorting? Just do it by date. Logins? Just a basic login, perhaps with tons of security vulnerabilities. Flagging? Later Comments? Not yet.
Once you have that, which is usually just a weekend job, then you can start to see what's going to be necessary and what isn't and start to see some of the edge cases and features you didn't even know you would need.