Thanks for the suggestion. I think the use of DNE here is because one of the common complaints we used to receive from students when we used problems from the OPL was the shifting syntax:
They found it frustrating to have to enter 'DNE' in one question, and then 'None' for the same response in the same context, in the very next question (and possibly other variations as well).
When we started building our own question bank we wanted to maintain consistent syntax so students could focus on figuring out the math rather than the interface.
Here I don't think we're taking lists as answers anyway: when there's more than one interval they're expected to use a union.