What the Washington treaty actually limited

Did the Washington Naval Treaty create the six-inch light-cruiser and eight-inch heavy-cruiser distinction?

No. Washington set a maximum of 10,000 tons and eight-inch guns for non-capital ships, without the later light/heavy cruiser split or a cumulative cruiser-tonnage cap.

The Washington settlement did not yet impose a total allowance on cruiser construction. A navy could build additional cruisers so long as each stayed within the individual displacement and gun-caliber ceilings. That freedom also weakens the case for placing heavy guns on the Lexington-class carriers because the same weapons could have armed another cruiser. The exchange avoids reading later treaty categories backward into 1922.