Indeed, very interesting. Upon a quick examination of mirc.exe, the phrase "FLOODCHECK" appears twice in the file. So without a doubt, it's a built-in function of mIRC. I really didn't know mIRC was smart enough to check for throttling.
On a side note, try sending yourself this same message...//msg $me FLOODCHECK
No message will appear, no TEXT events are fired. This only happens when sent to yourself, but appears normally when sent to/from other people.
If I might make a suggestion to Mr Khaled: Instead of sending this throttle detection packet via PRIVMSG, simply send a /raw FLOODCHECK and watch for the 421:Unknown Command response. This is universally compatible on all IRCDs, will not mess up one's idle time, and cannot be blocked by server-side ignore features (eg: +g).