No you can make a setting and have cowboys and still have it be unique. The issue i take with this is that shes building up the mc in a way that feels like hes a white savior. You can create a story but you need to be able to understand the meanings and implications behind what your writing. You can have a story take place after the civil war but you can’t treat its major implications as a side thought or plot device to move a story forward. If you don’t have an understanding of the setting then she shouldn’t be writing about it.
Honestly she could have not even included this stuff and would the story change? No. Thats how little thought seems to be put into the choice.

Could she not have just made up the setting like every other cowboy yaoi ever. Isn't this a bit tone deaf??