The author dedicates an entire paragraph to how much they trust the Obsidian team. It isn't open source purism, they are warning users that good intentions don't prevent a developer from writing software containing vulnerabilities.

Usage of user-created plugins and access to cloud accounts aggravates the risk posed by a vulnerability.

Open source reduces vulnerabilities over time, so those who want to heed the author's warning may indeed want to switch to an open-source Markdown editor. Just not because the Obsidian team is Evil.