The DOT rule does not require 5 year expiration for all credits. It only applies to compensation credits and credits for voluntary cancellations where there have been signficant schedule changes (3+ hour domestic, 6+ hours international -- which also qualify for refunds upon request per DOT rules). It does not apply to voluntary cancellations where there hasn't been a signficant schedule change to flights. It's possible they are being more generous than DOT rules require and are granting 5 year expiration anytime a flight has had a schedule change and not necessarily a signficant one. Flight number changes might also trigger 5 year credit expiration as it may be seen as a cancellation under DOT rules.

Note that Delta more explicitly spells this out on it's website -- https://www.delta.com/us/en/booking-...tes-gift-cards