It is not previous tier gear. It is the stock tier gear. For example, Tier 1 comes with standard Mk 0, which can be improved to a maximum of Mk II - Mk II being the equivalent to standard Tier 2 ships. So each ship comes with standard gear which can be improved up to the standard gear of the next tier ship.
It's not Commander stuff. It's Captain stuff. The point is you can improve upon the standard Captain stuff while you're a Captain. IE, you start with average gear and get better. If you started each ship with maximum gear there'd be nothing to work toward to improve your ship.