The thing about these courses, especially if they're about tech and advertised on twitter or reddit, is that they don't have to be directly about startups or making money. You simply have to reference money on your feed, and vaguely connect your course to this, and people buy. You can release a course just based on Rust, and people will still flock to it because your bio references making 50k a month or whatever the case may be.
Nothing will ever sell as much as the mere potential of the buyer becoming rich or attractive. There are entire billion dollar industries based on this. People have become NYT best sellers because of this pitch.
And so I'm not surprised at all your course has made substantially more than a serious startup product. While it's true, courses are perpetual, the ones with high quality content aren't really - they have to be rewritten and recorded over again with updated information. If this wasn't true, then MIT would have stopped posting newer courses online years ago. So these courses often have very little to do with content or sales channel, instead again it goes right back to this potential to become rich. It's a very very powerful motivator to spend. So powerful they spend, and never care about the content, hence being unwatched. They do it over and over again, making everyone else richer ironically.
So make sure you're well aware of that before making any crucial decisions about your startup, not all lessons are meant to be applied to it.