BHW Concurrency Question

I'm just curious if one of the veterans around here an shed some light for the sake of information. I see the concurrent active visitors at the time of this post is about 9,000 people. I'm guessing this is a LAMP built website. I'm curious as to what steps the webmasters have taken to scale up a php forum to that level of concurrency? Is this a stock stack or recoded internals of the forum? Is it just a cluster of hardcore VPS' with ssd and multi cores or does the servers not really matter and this has been recoded to be more of a static site? It appears templating is done on the server. Any thoughts or tidbits? I'm always curious as to see how sites running LAMP can scale so high of concurrency and not be terribly slow thanks.

It's in the footer.