Voltaire gets quoted so often without ever mentioning that the German word "Reich" has a different meaning than the loose translation "empire". A "Reich" is not necessarily an empire, so no news here.
It's true that the "Heiliges Römisches Reich" (literal translation "Holy Roman Realm") considered itself the successor of the Roman Republic. The English referred to the Roman Republic by what it was in the modern sense: an empire. Therefore its self-proclaimed successor got translated as "Holy Roman Empire", which, unlike the original, it is not.
Almost every single statement in this sentence was wrong. It was founded as the Sacrum Imperium -- literally the Holy Empire. "Heiliges Römisches Reich" was a later German translation of the Latin term for it. Charlemagne explicitly had himself crowned as the emperor. It was never in any sense a republic and it never pretended to be.
In the Latin phrasing, which they likely used, the word "Imperium" appears. Good point. Still, the original meaning of "Imperium" was slightly different than what it is today. So my point goes more like this: The Germans didn't claim to found an empire in the modern sense, so it's not that surprising that Voltaire pointed out, it isn't one.
Fair point, the original sentence is French and not English. Both are closer to Latin than German, so the mistranslation might be more on the German side.