"enabled" can be interpreted in many ways here. mIRC doesn't control whether IPv6 is enabled or disabled on your system-- that's up to your network interface layer. If you have IPv6 protocols enabled in one of your NICs, IPv6 is by definition enabled on your system-- similar to having SSL installed-- once it's installed, it's always "enabled".
The question is whether mIRC will *use* IPv6. The answer is: if priority mode is enabled, as often as possible. If not, it will only use it if explicitly requested via /server -6 or /dns -6.
Note that you can have a system without IPv6. You can also have servers that do not support IPv6. Therefore it is not *always* connecting with IPv6, only when it is enabled locally and on the server. This is not that common a scenario-- only a handful of EFNet servers support IPv6 as of now, for example. On other servers, mIRC will not connect using IPv6, even if you had priority mode on (because it would be impossible to do so), therefore the wording currently used is accurate.
