That understanding is wrong.
By enabling DNSSEC, you configure Pi-hole to...
Validate DNS replies and cache DNSSEC data
(quoting directly from the settings page)
Using DNSSEC validated replies takes additional CPU, memory and storage.
In addition to retrieving a requested DNS record, Pi-hole fetches signatures and keys to validate that the record has not been tampered with, discarding it if validation fails (for further details, see Understanding DNSSEC validation using Pi-hole's Query Log).