I think they're trying to say that if someone is likely to make just the one connection to your site, and then never fetch further pages, then you don't want to

I think they're trying to say that if someone is likely to make just the one connection to your site, and then never fetch further pages, then you don't want to turn keep-alive on. There would be no point maintaining the connection if the client isn't likely to use it. Imagine you run a site that provided a common javascript library. People are encouraged to add a link to fetch the .js file from their 3rd party webpages. So most clients contacting your server will be there to fetch one file only. Even if the user continues to browse the 3rd party website, they won't make further requests to your server. One connection, one request. Here, keepalives would be a bad idea.