That's exactly what the regulation means; you can keep fish under 12" long at Lake Malone. The statewide creel limit would apply to all fish of any size. Here's the whole reg:
• Largemouth Bass: 12 to 15-inch protective slot limit. All largemouth bass caught between 12 and 15 inches in length must be immediately released.

That means exactly what it says, and nothing more. Anything 12-15" you gotta throw back, anything else you can keep. If you weren't able to keep anything under 12" it would amount to a 15" size limit, and the "slot" would be meaningless.