The limit on the banner says that unit cannot use general's inspiring presence, i.e. if they were a leadership skink unit within 12" of the slann then they would have to test on Ld7, one more than their own Ld6 as they couldn't use the slanns inspiring presence.
If the banner is in a TG unit with slann (either as unit banner or BSB banner) then the slann's Ld is boosted to 10 as he is in the unit. Other units can use his new Ld 10 as they are not in the unit affected by the limitation. The unit cannot take advantage the general's Inspiring Presence Ld 10 because of the limitation, but can use the slann's Ld 10 as the highest Ld in the unit.
Because you cannot gain a desired effect through one rule doesn't prevent you from gaining the same effect through another rule. The limitation doesn't say you cannot use a general character's leadership, it just says you cannot use it via the Inspiring Presence rule.