I believe the sentence appears on this page that describes the behavior during a cold restart. It simply means the record’s ttl was never modified to a smaller value. The significance of a record’s ttl ever being shortened is that there is a possibility of the older, longer ttl version on the disk, reappearing although it has been superseded by a shorter ttl version if that version expired but was not written to disk. If a record’s ttl was never shortened, an older version will not be resurrected during cold restart.
For a more detailed discussion, you can read this article.