Two questions for you:
1. What ship of the line ever sailed as a privateer?
2. What ship of the line ever mounted 300 cannon, or even half that number?
I don’t keep up with such things but 300 does sound like an awful lot of cannons. Also my impression of “ship of the line” is something like what we called battleships which were in a line such as that required for crossing the T. In other words your most powerful ships. Probably a little expensive for a privateer.
Still his argument is sound. The Constitution provided for the private ownership of war ships.