I'd say the difference is convenience, brand recognition, and social media features.

You only need to sign up to Substack and you can already start publishing, so the vast majority of people who just want to write and not have to bother with building their own website will opt for the simpler option. Even if it takes only a handful more clicks to publish a personal website, the very idea of having to build something will be daunting enough to turn off most people.

Then there's the fact that while many people are willing to sign up to a well-known website like Substack, not that many are willing to enter their email into some random blog. I'm willing to bet that if some famous online personality made their own website+newsletter to publish their writings they'd get a lot of responses along the lines of "Who cares for antiquated personal blogs nowadays? What is this, 2005? Just make a Substack!"

And while the article presents Substack's social media features as a possible negative, the idea that anyone could see your post if it pops up in their frontpage, or that you might be the next lucky writer to get noticed by the algorithm and be recommended to thousands of people, will certainly be tempting to many.