The London Naval Treaty did not treat every submarine as a single undifferentiated type. Article 7 set the normal ceiling at 2,000 tons and 5.1-inch guns. It then allowed each treaty power three exceptional boats of as much as 2,800 tons armed with guns no larger than 6.1 inches. Those paired rules already supplied a legal template for recognizing cruiser submarines as a limited fleet category.
Chapter · 11:37-12:40
The London Naval Treaty already allowed large submarines
Article 7 limited ordinary submarines to 2,000 tons and 5.1-inch guns but let each treaty power retain or build three boats up to 2,800 tons with 6.1-inch guns.