This passage is an explicit counterfactual rather than a claim about the actual Washington treaty. It imagines separate limits near 20,000 tons for heavy cruisers and 10,000 tons for light cruisers, with higher capital-ship ceilings. Under those limits, the Royal Navy would probably remain below both maxima, around 18,000 and 8,000 tons, because slightly smaller ships would yield more hulls for worldwide stations. Ten 18,000-ton cruisers could fit into the tonnage that bought only nine at 20,000 tons, turning treaty arithmetic into operational availability.
Chapter · 3:06:38-3:08:46
What if naval treaties had allowed true heavy cruisers?
A clearly counterfactual treaty scheme shows why the Royal Navy might still choose 18,000-ton heavy cruisers and 8,000-ton light cruisers below the permitted maxima.