Think he is winding you up, but maybe not.
Diesel are ace for towing due to low down pulling power, BUT ford have a habit as with the cossie of not releasing details of towing weights etc for their more sportier variants.
I had this with my cossie they said "you cannot tow with a cossie, it is not safe", this is the biggest load of bull I have heard from a dealer, he then went on to tell me how other cars have uprated suspension and brakes, clearly he knew what a cossie was.
The most likely scenario is that ford may have done this with the ST tdci as well so this confuses some people who then think it is illegal, which of course it is not.
As regards it's capability to tow, would make ace tow car IMO.
tabetha