Check the graphs again, I'm pretty sure the Akrapovic says minus power. But the db killers are in, says so and you can see them.
I was perplexed when I read that site as well, I think either they made a publishing error on their own website, or the internals are steel and sleeves are titanium. Every system I see for sale says titanium. Their FAQ section refers to titanium vs carbon sleeves, and says nothing about stainless, so I think the internals are steel in either case and the sleeve is choice of titanium or carbon fiber. I emailed Akrapovic.
But it does refer to it as being the low-cost SP system, even though it doesn't look like it is one. Very confusing. On second thought, you may be right, it may be steel. But why is it $700 then? And why is it the lightest system in the test at only 3.5kg?