I think many people just don't realize that Udemy is a "marketplace"[1]. Yes, anyone can publish a course there, provided it meets some requirements. In the early days, Udemy's stated goal was to "democratize online education"[2].
There is an "Instructor Identity Verification Process"[3] which should make sure publishers own the content. No idea why it didn't prevent this case.
There is no "review by experts in the field". Whoever publishes a course is responsible for its content. If a course is inaccurate users will give it bad reviews and it won't sell. And in any case buyers get a 30-day money back guarantee.
There is an online form to report copyright violations[4], which looks simple enough to me.