Did you have expired points but were still within the 5 years to declare them or were they actually more than 5 years old?
It seems odd the DVLA had no record of the points however old they were.
"Just don't tell them" would seem to be bad advice as an insurance policy can be voided for not disclosing requested information.
They have no record since the points have expired. The DVLA have no need to keep record of points that have expired.