The issue about sharing your algorithms is minimal in this case. Most search engines are using standard frameworks. Google doesn't share their algorithms but StackOverflow doesn't add any specific magic to it.
Regarding how do you know where to route a query, it is an issue but not so great in this case. The article doesn't talk about having a two tiered search for every web site. If it has a two tiered search for the top 100 sites that is enough to challenge Google (the main point of the article) and making 100 searches and filtering them in the 2nd tier it's not difficult.
Regarding how do you know where to route a query, it is an issue but not so great in this case. The article doesn't talk about having a two tiered search for every web site. If it has a two tiered search for the top 100 sites that is enough to challenge Google (the main point of the article) and making 100 searches and filtering them in the 2nd tier it's not difficult.