The article is about security and trust. Open Source is in that context by definition the only good solution. Though, doesn't mean that a closed app has to be bad, but you have to blindly trust them, and hope that this will never change. With Open Source, you don't have to be blind, you can trust them educated (or at least trust that other will check what's going on).

Of course this always a bit of case by case, but obsidian is a very exposed and worthful target.