What a bizarre statement. Providing for security is also inherent to sovereignty of nations, but private security firms flourish.
It's also odd in the context of Germany in particular where a private currency has been used for many years:
https://en.wikipedia.org/wiki/Chiemgauer
I suspect there's a lot more behind this statement than meets the eye:
Inflation, and the ability of the state to force it upon its citizenry by forbidding the use private money - has come to be viewed as inherent to the sovereignty of nations. The world's governments are now entering a new phase in which none can afford to hold the more valuable currency due to the damage caused to export business. Net importers like the US have been holdouts, but even it too will have to bow to reality.
This conflict means that, no matter how well intentioned, policy makers will be feel compelled to devalue their own currencies. It's a race to the bottom, but without a bottom. Alternatives, especially private alternatives readily available to citizens, will increasingly be viewed as the enemy.