It's still reasonable to ask what's different about markdown-ts-mode compared with markdown-mode.

I read that article, and I didn't notice anything that stands out as different from markdown-mode (the non Tree Sitter version).

Regular markdown-mode also highlights everything, hides markup if you like, cycles section visibility if you like, fills paragraphs in bullets, etc. With poly-markdown-mode it also does language-specific syntax highlighting in code blocks, and even language-specific structural editing, automatic indentation, etc. With math-preview it displays TeX/LaTeX math as rendered formulae.