Why Scharnhorst's hull supports the fast-battleship label

Could the Scharnhorst class reasonably be called battlecruisers?

No. Their hull proportions and speed fit a fast battleship better than the longer, faster battlecruiser lineage represented by Hood.

Hull length provides an architectural check on the disputed label. Scharnhorst was longer than the older Renown class but remained far shorter than Hood, while battlecruiser hulls tended to lengthen more aggressively than contemporary battleships to obtain speed. A 31-knot target also sat within fast-battleship territory by the late 1930s. Classification therefore follows the whole design envelope, not the fact that Scharnhorst carried smaller guns than some rivals.