The video separates the missions rather than treating battlecruisers as badly armoured battleships. A battleship must absorb punishment, remain afloat, and continue fighting after heavy damage. A battlecruiser is a manoeuvre asset whose speed controls range, using that range as a form of protection rather than accepting a battleship's close engagement. Neither protection scheme is inherently mistaken when it matches the intended role, but the guns, shells, armour, and tactical assumptions behind it must rest on trustworthy performance data.
Chapter · 24:47-26:54
Battleships and battlecruisers were built for different risks
A battleship was expected to absorb punishment in battle, while a battlecruiser relied on speed and range; each design protected itself according to a different mission.