No, this is not a premature April fool's joke, there really is a Basil Woverton drawn Mickey Mouse. I'm as surprised and awestruck as you.
Thanks to Comics Comics for the discovery.
This single shot was promoted to show the glass ceiling of subjects the Batman animated series was allowed --which you can see by turning upside down--, although it was still a little looser than 1980's cartoons. Also, it's a really nice, well constructed picture (courtesy of Bruce Timm), which I always appreciate.
1) His own words: The infamous 1978 interview
2) A sample of his body of work (covers only)
Stuff I like; no more, no less