'all leather' interior is right?

At Porsche, anything was possible if enough money was paid for it. Today they have a special division that supports custom requests, including leathers, colors inside and out. I do not know if that division was in place back in 1979.

Full leather interiors were more commonly installed in European cars, the cooler climate and less sun did less damage. Porsche realized early that the US market was problematic for leather so they didn't ship that many cars with full leather to reduce the chances of customer complaints.

So yes, it is possible that a 1979 Porsche 928 had a full leather interior. Does the leather condition match the other interior bits? If it looks considerably newer than the rest, it probably was installed later.