I've done some digging on this
@Sean d . You are essentially correct but it's a twisted plot.
One correction first of all. Mohair is not a type of denim. It's unquestionably from a goat.
https://en.wikipedia.org/wiki/Mohair
Does that mean our roofs are made of goat hair? No. Why not? Because despite every soft top supplier around telling you their hoods are Mohair, including Jack Smith they are not. These guys make a very clear claim of mohair which is totally contradicted by their supplier.
http://www.thesofttopshop.com/bmw-z3
This page from their supplier appears to give the truth.
http://www.haartz.com/vintage-restorers-guide/glossary-z
In summary from this page:
The term “mohair” is still used in the United Kingdom to refer to rubber-combined cloth “hood” (top) fabrics, although we are not aware of any such material containing true mohair.
Assuming Haartz supply everyone and not just The Soft Top Shop. Reasonable assumption? This is what the claimed Mohair actually is.
http://www.haartz.com/exteriors/cloth-topping
It's one of 4 possibles - I suspect Twillfast (certainly The Soft Top Shop use this) but they are all essentially heavy duty cloth finished which you may want to call denim.
Does any of this matter? I think probably not. It's not just a piece of denim. if you read the Haartz descriptions you'll see that this is a highly engineered multi-layer covering used by the factories of all the top car manufacturers in the world.
So in short an unnecessary piece of marketing to say it's Mohair. But for sure what Jack Smith uses looks good and seems to hold up well.