It'd be selection bias if he made the assumption that most big businesses were using Ruby. That's not the point he was making -- what he said was that lots of people using Ruby work at big businesses.
No, you don't understand what selection bias means.
Selection bias is when a developer from Big Biz is more likely to go to conferences than a developer from smaller business. Do you have any specific reason to believe that? Not knowing much about conferences, I don't. So until you tell me of those specific reasons, I have no reason to believe in selection bias.
Now, there's another sampling effect: maybe Big Biz simply hire (much) more developers than smaller businesses. If 5% of Big Biz devs work on Ruby, and 10% of startup devs work on Ruby, but 90% percent of developers work for Big Biz, then your Ruby conference will still be 80% full of Big Biz devs without selection bias, even though Big Biz is twice as hostile to Ruby.
They have more money so they can send more developers to these conferences than startups. Also, they have more employees, so the odds of a number of them being Rails developers is higher.
But that's not even what was being discussed in the context of my "selection bias" remark: the OP was saying that Ruby is used in BigBiz and used Ruby conference attendance to support this claim, which is different from what you are arguing.
On top of that, the initial statement was "almost no big business uses Ruby".
Selection bias.
Go to a COBOL conference and you'll likely find that most attendees work in Big Biz as well. What does that tell you?