Yes, quite right. However, the Graf Spee and sisters were never in the Naval treaties and were conceived to sail through the loophole created by Versailles. Meanwhile, the whole heavy - light cruiser distinction didn't become an issue until the London Naval Treaty in 1930. That was where they actually limited the number and total tonnage of cruisers with 8" guns (heavy cruisers). It invented a new category of cruisers with guns under 6.1" (155mm) which became light cruisers. This was also the treaty that defined destroyers.
ARTICLE 15
For the purpose of this Part III the definition of the cruiser and destroyer categories shall be as follows:
Cruisers
Surface vessels of war, other than capital ships or aircraft carriers, the standard displacement of which exceeds 1,850 tons (1,880 metric tons), or with a gun above 5.1 inch (130 mm.) calibre. The cruiser category is divided into two sub-categories, as follows:
(a) cruisers carrying a gun above 6.1-inch (155 mm.) calibre; (b) cruisers carrying a gun not above 6. I-inch (155 mm.) calibre.
Destroyers
Surface vessels of war the standard displacement of which does not exceed 1,850 tons (1,880 metric tons), and with a gun not above 5.1-inch (130 mm.) calibre.
It also limited the parties by tonnage within these categories.
CA (>6.1") CL (<=6.1") DD (<=5.1")
British 146,800 tons 192,200 tons 150,000 tons
US 180,000 tons 143,500 tons 150,000 tons
Japanese 108,400 tons 100,450 tons 105,500 tons
London Naval Treaty of 1930
Washington Naval Treaty of 1922