It's not a legal defense at all.

Licensed professionals are required to review their work product. It doesn't matter if the tools they use mess up--the human is required to fix any mistakes made by their tools. In the example given by the blog, the financial analyst is either required to professional review their work product or is low enough that someone else is required to review their work product. If they don't, they can be held strictly liable for any financial losses.

However, this blog post isn't about AI Hallucinations. It's about the AI doing something else separate from the output.

And that's not a defense either. The law already assigns liability in situations like this: the user will be held liable (or more correctly: their employer, for whom the user is acting as an agent). If they want to go after the AI tooling (i.e., an indemnification action) vendor the courts will happily let them do so after any plaintiffs are made whole (or as part of an impleader action).