It shows up in hours on github because github has a lot of authority. It shows up high on google because I'm guessing you're searching for pretty long keyword strings. So if your ok with people finding your code when they search for "ruby gem for mysql connections" thats cool you don't need to hire an SEO professional. Hell you never need to hire an SEO professional. But if you want someone who knows more about it to help you with making sure Google and yahoo and bing can find your information then you can.
I really don't get the hatred on this site for SEO in general. I'm not an SEO professional myself but I know quite a few and now a days most people in the profession that are active and well known are all above board. They help you structure your information to make it easier to find and help you build relationships with other sources of traffic. Hardly evil stuff.
The goal of SEO, good SEO, is to build a site that caters to both users AND SEs. After all, users make you money, and ignoring one over the other is silly.
A classic example is AJAX. SEs can't pass it, and neither cana lot of users. A "good" designer may know this and buid a site to cater for these users, but how? Will it use forms (invisible to SEs) or an alternate structure?
Sure, a great web designer may "get" all this, but can you be sure? Given that SEs provide the largest, ongoing form of traffic online, why take the risk?
"I make something, like some small open-source software, put it on GitHub, and it shows up as the first result on Google within hours if you are searching specifically for something like it. I didn't have to hire a SEO."
Fallacy of dosage: ranking for a service with little competition is pretty easy. Capturing the whole raft of traffic available on a topic: difficult!!