This is, of course, a matter of opinion.  My own view is that Voltaire was largely correct when he said this.  There are three aspects to Voltaire’s statement.  He is saying that the Holy Roman Empire was not holy, that it was not Roman, and that it was not an empire.  Let us examine all three aspects of this statement.

First, was the Holy Roman Empire holy?  It is hard to imagine that any worldly kingdom can be truly holy given the things that people have to do to stay in power.  The Holy Roman Empire was certainly not holy.  For example, the Holy Roman Emperor Henry IV was excommunicated by the pope for his role in the investiture controversy.  In this controversy, the emperor wanted to keep appointing high church officials as emperors had been doing in the past.  Appointing church officials on the basis of politics does not seem like a holy practice.

Second, was the empire Roman?  It is once again hard to say that it was.  Of course, the emperors were crowned by the pope.  This could imply that Rome was the center of the empire.  Also, the empire was supposed to be a recreation of the Roman Empire and Latin was its lingua franca.  But the empire was not truly centered on Rome.  It was more of a Germanic empire that really only paid lip service to the idea that it was Roman.  It was not truly a remaking of Rome.

Finally, was the empire an empire?  I would say that it was not.  The Roman Empire was truly an empire in that it was ruled quite strongly from its center.  At most times in the Roman Empire, the provinces were not in any way independent.  By contrast, in the Holy Roman Empire, the emperors lacked the power to enforce their will throughout the empire.  Your text mentions that the popes would make alliances with various regional powers to try to check the power of the emperors.  In other words, the Holy Roman Empire was more of a loose confederation of states whose emperors could not truly dominate the regional powers. 

In these ways, I would argue that the Voltaire’s statement was true.