Teaching the topic to students is the best way to learn it thoroughly because it closes the loop and you are challenged from many different angles. Something you understand weakly will be something you explain poorly, and be a big sticking point you'll need to revisit repeatedly.
The blog approach might get a similar benefit if you can get a lot of feedback and discussion for each one. But it seems like it would have a degree of self-selection of readers, where you only get the most motivated commenting on any given post.
I have to say I think you're right. I think the advantage of a blog format is that it forces you to capture your thoughts in writing. So you have to challenge yourself.
But, having taught some in-person and synchronous online classes, the different points of view from students definitely force me to understand the topic at many levels.
One issue teaching might have that blogging doesn't is that it may be hard to find your audience, if it is a niche topic. For instance, I doubt that anyone would have signed up for a Cordova automation course (the topic of my ebook). But perhaps in a world of Udacity I'm incorrect.
The blog approach might get a similar benefit if you can get a lot of feedback and discussion for each one. But it seems like it would have a degree of self-selection of readers, where you only get the most motivated commenting on any given post.