Another valuable role I've discovered is that of the incubator (I'm not YC; doing OTBC in PDX). It's another secondary role similar to Lawyer, but can be very supportive in making sure you're focused on doing all the business stuff that hackers can easily lose sight of. I'm presently being shown the value of having a solid business plan, perfecting multi-length elevator pitches, and refining the more formal investor and sales pitches. This is all possible thanks to experienced entrepreneurial coaches who really want to see you succeed.
Lastly, the "consort" is a non-obvious role -- someone who can give validation to your thinking, and exchange predicament stories with you. You may find him through the incubator, or at a hacking event, or wherever. My main consort is an extremely capable hacker and our regular discussions are gold.
So with the core team containing the right hacking, creative, and outreach balance, you can make progress. But I want to stress that the supporting people (Lawyer, Consort, Coach, et al) provide a lot of necessary elements.
He introduces the list as "...that I see as important to making a new site come together."
Not "startup", "site."
I could be wrong, but Twitter doesn't need to focus on lead-gen...
One could argue that viral marketing is business development. The main difference is that in the former, your customers are doing the work for you, while in the latter, you've got a defined role.
I'd be careful to suggest though that this role isn't being done at all.
I think the author is suggesting that the "Marketer" fulfills this specifically in the vertical he's approaching.