Here's the relevant original text from the German law they made to implement the TPD2 directive
5. Wenn Einweg-E-Zigaretten und Nachfüllbare Tanks sowie Einwegkartuschen mit Nikotinhaltigem Liquid verkauft werden, dürfen diese max. 2ml Liquid Fassungsvermögen haben. Diese Volumenbegrenzung gilt also nicht für Nachfüllbare Tanks, die ohne Nikotinhaltiges Liquid verkauft werden.
and my translation
"If disposable e cigarettes and refillable tanks as well as single use cartridges are sold
with eliquid containing nicotine they shall not exceed 2ml of capacity. This limit does not apply to refillable tanks sold without eliquid containing nicotine."
So in a sense I was wrong (the definition of tank doesn't seem to matter), the important factor in Germany is if the tank/cartridge was sold with nic liquid. If it's sold empty there is no volume limit here (as there isn't in several other countries as well).
In the end it was the UKs law that limited tank size to 2ml, not the directive itself.