SK IS ALLOWED, and the mod simply has no idea what they are talking about, Just a mis-communication.
Here is proof in Runescape's own rules:
Support > Other > Fansites > Section 8.
8. Can I use third-party RuneScape software?
We do not recommend players use third-party RuneScape-related software of any sort. Please be aware that there is software out there which claims to be a cool RuneScape add-on, but actually installs a keylogger on your computer and steals your password! You should also be aware that there is dangerous software around that won't necessarily be spotted by your anti-virus software. Scanning downloads is, of course, a very good idea, but it still doesn't 100% eliminate the risk.
Furthermore, if such software gives an unfair advantage, encourages rule-breaking, or endangers users' accounts then its use is not allowed under the Rules of RuneScape, and using it may result in your account being banned. If in doubt, don't use it!
Therefore, If the 3rd party client does not promote INGAME user advantage, then it is not against the rules.
SK has nice features such as IRC and screenshots, which are advantages, but not INGAME advantages
, therefore not against the rules.
Unless MMG comes out when a statement on the main page saying SK is against the rules, we can assume its perfectly legal.
EDIT: Jagex will not promote SK however. If someone asks if a 3rd Party Client is against the rules, Jagex will say yes, even if its not specifically illegal. This is simply because they are against all 3rd party Clients regardless.