Then you missed the core point of the article, so I'll try and rephrase it:
While you're staying in Markdown-only land (the left part of the article's graph halfway down the article: low-medium richness (horizontal), very low complexity (vertical)), you are not learning HTML.
You miss the "onboarding" process to the richness of HTML, so when you reach the limits of what Markdown can do, and suddenly you need something medium-rich... you have to play catch-up to learn all the HTML that you avoided by sticking to Markdown. That's the gap.
The fact that Markdown supports in-line HTML allows you to bridge the gap, but is irrelevant to the point.
Then you missed the core point of the article, so I'll try and rephrase it:
While you're staying in Markdown-only land (the left part of the article's graph halfway down the article: low-medium richness (horizontal), very low complexity (vertical)), you are not learning HTML.
You miss the "onboarding" process to the richness of HTML, so when you reach the limits of what Markdown can do, and suddenly you need something medium-rich... you have to play catch-up to learn all the HTML that you avoided by sticking to Markdown. That's the gap.
The fact that Markdown supports in-line HTML allows you to bridge the gap, but is irrelevant to the point.