1. Even if it's a paid course, that doesn't mean that all students paid money. Udemy instructors have the ability to give out free coupons to help in marketing their courses. Since it's in Udemy's best interest to boost their displayed course enrollment numbers, signing up a free account and 'purchasing' a course for free is really easy. I wouldn't be surprised if some instructors use a script to automate this process and enroll thousands of fake students.
2. Even if people do pay, almost no one pays full price (and the price you see may not have been the price at launch). Udemy gives frequent and deep discounts ("50% off all courses for Udemy's 4th birthday!", "70% off because summer's out and the school year ended!", "90% off select courses just because it's October!"). Many Udemy users are 'course collectors' - they'll purchase a bunch of courses at once when it's on discount, in case they may want to learn those skills in the future.
3. Udemy gets a bigger cut than you think. While instructors keep 100% of revenue from customers they bring to Udemy, the majority of your students will be Udemy users you don't know. For these purchases, instructors receive 25% or 50% of the purchase price (depending on whether Udemy did any marketing), with an additional discount if the course was purchased on a mobile device.
Don't build a course to 'get rich quick' - do it only if you actually like teaching.