Evelyn,
I'm not sure why he is listed as buried there, in his time that has a Memorial site for the unknown unnamed Indian/Soldiers-Confederate-unmarked.
It may of been his wish to be buried there we may never know, or perhaps his family request.
I would think a newpaper of that time,would have possibly wrote of his death, however being buried there is a differant matter.

There is no proof of any one in any of the unmarked grave site/of this Fort Washita (site) other than the marked one stone of {Col. John Johnston}

It's unclear that SCV or UDV or anyone can prove any other listing --I THINK NOT-- My opionion