It is but at the same time it's not.
If you make a course in a category where there's 500 other courses which is the norm for a lot of tech niches you'll be on page 30 and no one will ever find you. For new courses Udemy might decide to show your course to a tiny percent of traffic in a higher search ranking to see if you gain traction but unless you drive your own traffic to Udemy from your own site / resources you're going to have a hard time competing with folks who do. Their algorithm very much prefers instructors who send their own audience to Udemy.
If you don't gain that initial traction you're basically dead in the water. Plus it's going to be a hard sell to get someone to sign up for your course with 4 enrollments and no review vs courses who have 100,000+ enrollments and a 4.4+ rating.
With that said, having no audience won't help much on Udemy vs being on your own platform. You're still going to need to grow an audience and send traffic to your course landing pages no matter which platform you use (a 3rd party marketplace or your own site).
I started on Udemy with 0 people in my audience, and I still have 0 people outside of my Udemy audience. I don't do any marketing or drive any external traffic.
When you launch your course, you have a 30 - 90 window that you get scrutinized on. The biggest metrics the system is looking for are thumbnail CTR, enrollment rate (%), total minutes watched, and average rating. For most courses, the system doesn't need a lot of time to figure out that your course is not better than ones it already has (fewer people click it, fewer people are compelled to buy it, the ones that do watch less, and then fewer people feel compelled to rate it).
No, they do NOT heavily favor people who drive their own audience, that makes no sense. They sell their courses for $10-20, so no one in their right mind is sending large amounts of hard earned email traffic to such a low ticket sale. You can literally go into the Marketplace Insights tool and see where they list the % of sales from "Instructor Promotions". It's rarely ever over 5% of total sales and most of that is from internal promotion emails to your other course students. One thing they DO favor is instructors who have multiple courses, because they know that your pool of students is more likely to buy other material from you so its an easier sell for them (remember, they spend $ to acquire the student and they do not breakeven if the student only buys 1 course).
Talk to any successful Udemy instructor and they'll tell you that no they do not drive their own traffic. If they could drive that much traffic, they would make their course independently, charge a lot more, and make way more money.
If you wouldn't mind reviewing https://news.ycombinator.com/newsguidelines.html and taking the intended spirit of the site more to heart, we'd be grateful.
I heard a podcast recently and a successful Udemy instructor had said that these days, the SEO required to succeed on Udemy is even harder than the SEO to rank in Google. When you search for a topic on Udemy (eg. React, Go), there are dozens of highly-reviewed courses, and it's impossible to show up in those results unless you already have a huge audience or want to spend lots of money on paid traffic.
So yeah, I think it's true that you need an audience to succeed with a self-published course, but honestly I think that's true no matter which route you take.
The vast majority of sales are from their personalized recommendation widgets and email blasts. No SEO optimization needed for that.