Traditionally furs were used as ground insulation. When people became interested in travel by foot rather than by horse, we see the other options listed by Kephart. None of them are great. If you pile up enough of any insulation it will keep you warm. Carrying is going to be a problem though. There is nothing that can directly replace the Synmat in terms of insulation per weight ratio.
I think this statement is wrong. And the whole notion of traditional. Traditionally people didn't travel out of anything else but pure necessity. This travelling in nature as a pasttime is a rather new thing which came with the novelty of leisure time (and money to consume stuff they actually didn't need to survive or improve the farm).
Travelling by horse has never been a viable option for most people, unless you were a hun. People have always been walking, while shelter and supplies has been carried by horse or other pack animals. In more meta-civilised times people walked to a large body of water and took a boat. And most often travelling using the large networks of hostels, taverns and whatnot that provided roof above the head and warm food for the weary traveller. It's not by chance that travelling literature spellbound large audiences since mr Gutenberg started his machine, people were just not travelling in any greater extent. Not in the way we think about travelling at least. The journey man system made people moving around but usually not with any need for camping equipment. If somebody were far from hostel or similar they would just sleep in a barn or whatnot.
But yes, furs have been used for insulation and domestic bedding for as long as man has been on this earth. And, natural insulation is not as weight efficient as modern materials. But you may have noted that most people here does not give flying eff about weight effectivness. Nor about any other effectivness. So it's time to face that no matter how much you preach about modern materials, people here will not care about the technical specs. You would be better stating the obvious on a ultralight backpacker forum.