Indeed, mIRC has supported passive DCC for quite a while, even there just was no option to actually turn it on for normal (non-firewalled) connections. See for example my post about the passive DCC protocol
- this is from before 6.17 was released!
To the OP: it's documented in the helpfile under "DCC Socks5 Protocol". I'm not sure when that page was added to the helpfile.