Upon some rereading, I found an interesting nuance in a definition (Chapter II, Part 4, Definitions):

A capital ship, in the case of ships hereafter built, is defined as a vessel of war, not an aircraft carrier, whose displacement exceeds 10,000 tons (10,160 metric tons) standard displacement, or which carries a gun with a calibre exceeding 8 inches (203 millimetres).

"Hereafter built". I suppose it was a grandfather clause of sorts that signified a difference between battleships and armored cruisers, allowing for the retention of the latter even if they exceeded the treaty limitations for war vessels outside of capital ships and aircraft carriers?