I’m not sure I understand. You say it’s been a month, so you got a response in early December, despite applying ED? If I understand it correctly, all ED decisions from schools must be given out by Oct 20, otherwise they must release your application to apply to different schools.
As for the deferral, usually it simply means they are offering you admission into the class starting fall of 2019, but they may offer you earlier admission if they burn through the waitlist and still have spots open in the fall 2018 entering class. Did they request a response from you? If so, what was the deadline and did you respond?
I’d recommend contacting the school to discuss your questions.