Only if you accept the premise that the CSA constituted a legitimate separate nation. If you maintain the fiction that they were merely a "rebel" section of a single nation, than any activity by any ship under their control constitutes mere criminal acts.
As I have said several times before. The "condition" of the CSA is in a quantum state of superposition, being both legitimately a separate nation, and simultaneously a rebelled section of a single nation, depending on the need of the winning government to paint it as one thing or the other.
It's status is completely subjected to the instantaneous needs of the victor, and could be changed as necessary to fit whatever rationalization argument they wished to put forth.
Semmes was indicted in 1866 on charges of treason. Those charges were later dropped. Semmes was not indicted for piracy. That would indicate that the senior officials of the Johnson administration recognized that he was acting within the accepted bounds of Naval warfare. The Royal and the French navies seemed satisfied that Semmes was acting in that manner also. Had they believed otherwise there standing orders would have allowed them to pursue the Alabama as a pirate.
Alabama vs Kearsarge published in the Burlington Weekly Hawk-Eye (Burlington, Iowa) newspaper