That is not the provenance (origin) of copperhead, it is another meaning of the word though.

https://www.merriam-webster.com/dictionary/copperhead

The first known use of the word "copperhead" was in 1775, well before the US civil war.

This is a copperhead.

https://en.wikipedia.org/wiki/Agkistrodon_contortrix

Being from the south I've occassionally heard and used the term "carpetbagger" which doesn't have slavery connotations that I know of but is from the same era. I've never heard the term copperhead used with that meaning, maybe only people from the north know it?

No one uses that definition anymore.