Why is the resonance frequency of an undamped oscillator equal to the undamped resonance?

I have read this post: 'How do you define the resonance frequency of a forced damped oscillator?'

And I see that the resonant frequency occurs at the undamped oscillation frequency $\omega_0$ as opposed to the damped oscillation frequency $\omega_d$. I don't understand why this is the case though? In the post, it stated that at resonance the ' energy flow from the driving source is unidirectional', and I'm sure this is the reason why it is the natural frequency of the system not the driving frequency at resonance, but I didn't really understand the rest of the post to see if it answered this question.