Even if we ignore for the moment the vintage of the tartan, I think the reason you will see modern accessories with a purportedly historic uniform is two fold.

First, since there is little if any record of confederate kilted units, there is little if any source material to work with. No drawings or photos from the period that would guide you down the right path. So you can find a great deal of stuff about jackets, you can find a great deal of stuff about kilts in other settings, but the two do not come together in any sort of educational resource. I can not explain why someone would not look at a non confederate picture of highland wear from the same period but I can understand how it happens.

The second is a matter of availability. For the casual kilt wearer, with little more than the internet and maybe a few festivals to go on, one will not find a historic sporran. In the USA unless you see a touring band the only source material you have for kilted uniforms are generally pipe bands, and they are wearing modern accessories for the most part. I know that I did not come across anything other than a few pictures in books that would show the correct time period until reading about the issue in various threads here on XMTS. I doubt many of the kilted confederates are reading through various non confederate threads on a kilt forum. Even if they did, finding one of these period correct sporrans for sale can be a daunting task. Not many of the well known (or lesser known for that matter) kilt shops carry such an item.