Ah but for the feat in question , he was explicitly using his muscle power as opposed to unbreakable skeleton or otherwise
http://static.comicvine.com/uploads/...8843-eleva.jpg
Even if it had been his skeleton alone giving him his peak human stats , it's well ....peak human stats in the end . Peak human stats + unbreakable claws seems good enough to cut steel for me
Personally I like to think of that as Cap having enough skill to throw it to " cut " or " not cut" every time he's going for slicing or otherwise
Also you will notice that
I am not using it as a magical object that gets a free pass for when it hurts say Hulk or Thor
But cutting through a truck or a tank? Seems oKay for someone who casually benches a 1000 pounds