Yes, I said few.
Even if in my blog post about Debian's ever-growing mirrors network it seemed like we have a lot of mirrors, truth be told: they are too few. There are many partial or even complete mirrors out there that are not listed.
If you are the administrator of one of those mirrors, please consider submitting your Debian mirror. It helps keep track of them and expose them to more users.
Granted, there are some that due to policies (on remote connections, bandwidth use, etc.) they are kept private. Hidden behind a LAN, hoping that people inside actually use them. As of this time, there's not much that can be done about them. If they were to be listed somewhere, there's no way for somebody to check them from the outside.
Some other mirror networks require that an IP (or a range) be whitelisted, allowing them to access the mirrors from the outside to perform routine checks. For some reason, I thought Fedora's was one of them but, apparently, they use a different approach: they ask private mirrors to run a tool.
I think that each approach has its pros and cons.