I understood GP's point to be that because Obsidian leaves a lot of functionality to plugins, most people are going to use unverified third party plugins. On arch however most packages are in core or extra so for most people they wont need to go to AUR. They are more likely to install the flatpak or get the appimage for apps not in the repos as thats much easier.

yay or paru (or other aur helpers afaik) are not in the repos. To install them one needs to know about how to use AUR in the first place. If you are technically enough to do that, you should know about the security risks since almost all tutorials for AUR come with the security warnings. Its also inconvenient enough that most people wont bother.

In obsidian plugins can seem central to the experience so users might not think much of installing them, in Arch AUR is very much a non essential component. At least thats how I understand it.