The "non-refundable" language is just fare rule boilerplate taken from non-refundable revenue fares. It doesn't have anything to do with the ability to cancel and redeposit miles from award bookings which is controlled by DL policy and is not covered in the fare rule language. They waived redeposit fees during Covid for all awards (formerly, there was a $150 fee unless you had Platinum or Diamond status). They brought back fees for Basic Economy awards in 2022 in the form of mileage deductions (9900 miles domestic and 19900 miles international), but have continued to waive any redeposit fees on Main+ awards no matter your status level. There is still some language about $150 redeposit fee for awards that do not originate in US/Canada at this page -- https://www.delta.com/us/en/baggage/overview ("A redeposit fee of $150 will still apply for Award Tickets and mileage upgrades for travel originating outside of the United States and Canada.") But this does not seem to reflect current reality.
Note that when booking roundtrip awards, once travel has begun, they do not allow redeposits of miles. https://www.delta.com/us/en/skymiles.../program-rules ("Once travel has commenced on an Award ticket, the Award is considered used. Mileage will not be credited back to the account for the unused portion of the ticket.")