Flying Blue Points Expired...Question

Hi,

I had my Flying Blue miles expire despite what I thought was qualifying activity. I booked a Delta flight using Flying Blue miles, and but my points expired anyway. I emailed Flying Blue and I got this as a response when I questioned why my miles expired:

"Thank you for contacting Flying Blue. Activities related to purchases made with Miles or completing a flight paid with Miles do not extend the validity of the Miles in your account."

Is this true? I thought a flight booked with FB miles does extend the validity of miles in my account. Anyway, I'm just looking for clarification on this. They did offer me a chance to reinstate my miles if I purchase in cash a flight on Air France or one of their partners by a certain date.

Thanks for your help clarifying this.