Instead of injecting into the builtin Markdown grammar, this extension overrides it completely (even though the grammar is still mostly the same as the one built into VSCode, with a few tweaks). The reason for this is that I also use these grammar definitions for parsing Markdown in Markane language server via vscode-textmate library, and vscode-textmate doesn't support language injections.
See also
Document Preview VSCode extension for previewing Markdown via Pandoc