The thought here is your persona in a given space is way more valuable than the money you will make off selling a course or paywalling your content, and instead should open it up to as many people as possible.
In thinking about this, I think it may be dependent on how niche your audience is. If you have truly unique and differentiated content and your audience is niche, then you may be better off paywalling it.
I think the counter-argument here is you still need a monetization strategy of some kind.