The attrition rate is indeed sky high but it'd be wrong to say they require a CS degree or equivalent. In fact, the high attrition rate indicates the exact opposite - that they admit a lot of people who shouldn't have been admitted in the first place.
The OMSCS subreddit has active admissions threads and there are a fair number of non-STEM people who get into the program every year:
The OMSCS subreddit has active admissions threads and there are a fair number of non-STEM people who get into the program every year: