The thickest plate is only one measure of an armour scheme. Belt height, belt length, and the volume enclosed behind it determine how much of the ship receives that protection. A battlecruiser can therefore quote battleship-level maximum thickness while covering a much smaller area. It is the difference between a small chest plate and full body armour made from the same material.
Notable Point · 3:30:19-3:31:24
Battleship-thickness armour is not battleship protection
A battlecruiser can match a battleship's maximum belt thickness while protecting far less of the hull.