Hayes, the term "bushwhacker" seems to have been pretty indefinite -- sort of incident-specific. In the context of the 6th Kansas Cavalry letters, it meant a guerrilla -- hit and run attacks on Federal troops, sniping from places of concealment, etc. Their effectiveness is apparent from the description of the extreme lengths the Federals in Arkansas and the Indian Territory went to villify and summarily execute them.

That particular description doesn't seem to apply to the man you cited, though.