It won't work the triceps any harder than a regular bench press, assuming grip width remains constant on supination versus pronation.
Here's where the myth comes from.
When you pronate your grip (overhand), you more actively engage the pectorals - particularly the sternocostal head. The clavicular head is also engaged - hence, why people say bench pressing is a good all around pec builder.
However, when you supinate your grip, the sternocostal head is less actively engaged. The clavicular head, of all things, actually is preferentially recruited versus the sternocostal head (so yes, flat reverse grip benching is one way to help target upper pecs).
So, relatively speaking, pound for pound, a reverse grip bench will engage FEWER pec fibers than a pronated grip bench press. Does that make it a better triceps builder? No. But it does make it a LESS EFFECTIVE pec builder.
It's just the logic that got scrambled here.
Don't believe me? Hold your arm up in front of you, and grasp your triceps. Now, pronate, then supinate your arm. Feel the triceps flex differently? No, you didn't. That's because triceps aren't preferentially recruited when arms are supinated or pronated. So reverse versus regular benching doesn't help activate triceps more or less. It's just more or less (or different) pec involvement.
