They definitely do not claim they are "fully refundable". They just copied the rules for BE revenue fares when booking, instead of BE awards, and says "cancel for partial e-Credit" at booking. It shoud say partial mileage credit with 9900 mile deduction for US/Canada awards and 19,900 for long-haul international.
This was announced way back in 2021, but is apparently not being enforced according to several reports.



https://pro.delta.com/content/agency...c-economy.html




https://www.delta.com/us/en/baggage/overview