Now this is strange... Bilstein Europe is indicating mono tube shocks for their 986 aftermarket struts
Parts catalogue
They are advertising "B6 Sport" and a "B8 sprint" aftermarket options on the Bilstein Europe catalog.
(If you actually get to looking at a Bilstein strut, usually whether it's twin tube or mono tube is pretty obvious as the monotone is usually mounted "upside down" with huge-looking shaft exposed (because that's actually the tubular part of the mono tube).
Might just be a case of the U.S. Bilstein catalog being transcribed incorrectly. I'd think a call to Bilstein tech support should be able to clear up what the design of their aftermarket product actually is.