verb agreement - Is this mixture of plural and singular legitimate?

But what is most important for our purposes is that these changes were
the signal for the resumption of historical debate on a grand scale,
of the kind that had been suspended or driven underground during the
reign of Louis XIV. （The French Idea of Revolution ed. by Dale Van
Kley: 203)

I am interested in the expression "these changes were the signal". Shouldn't every change be a signal, so that there are many signals? Why use the singular form? Because all these changes together form a signal, and one or two just cannot signify that there is a resumption of that debate?