While I do like the aesthetics of this, as others have mentioned it is plain wrong.

Any explanation of resonance should deal first and foremost with a physical system, not a signal. Having two signals 'resonate with each other' does not make a lot of sense.

Not sure what is meant with multiplying because the author then goes to mention integration which in essence, is adding, not multiplying.

A physical system is able to store energy at specific frequency -- a pendulum will swing for a long time, energy slowly decaying. But if the system is excited (imagine a kid on a swing), with each push, we will add some more energy, which will accumulate each time adding to a large response (resonance). The key part is, we need to add energy at the right frequency, push at the right interval, sing with the proper pitch.