You would call someone like this an "armchair expert". An armchair expert is somebody who speaks with authority on a subject while having little to zero practical experience in the topic at hand. The number of people acting like armchair experts appears to be increasing, and it appears to be in direct correlation to the ever increasing availability of information via the internet.
Thanks to the internet, I can read endlessly about medicine and medical problems. I can casually fill my head with half facts and regurgitate them on demand. However, being that I am not a doctor, though I could shoot my mouth off all day regurgitating these facts, I would most likely fail entirely at accurately assembling or interpreting any of this information. To other non-doctors however, it may appear that I am accurate.
The moment I actually attempted to do this, I would be an armchair expert. It would be hot air making me sound like a medical expert, masking the fact that I'm a complete fraud, a counterfeit masking the magnitude of my incompetence with a false air of confidence.
Another example of an armchair expert is all the men that sit in bars watching UFC shooting their mouths off criticizing a fighter, while the only fighting experience they have is getting a wedgie by the school bully in grade 6. These are all armchair experts. I hope you had a laugh. ;)