SK actually advertises this flexibility, so you should be fine. The fare rule/clause you read is about the kombi construction. So if the outbound or inbound is constructed with two fares, the most restrictive apply.

Say you are travelling LLA-ARN-LHR-ARN-LLA and the fare is priced LLA-ARN (like WFIX)+ ARN-LHR(say YFLEX), the most restrictive of the two (WFIX) will apply to LLA-ARN-LHR.

Clear as mud?