People keep saying this as though it weren't apparent. Of course he's stupid. He's also boastful, prideful, fueled by ego and has literallt lived 70 years never having to face consequeces for what he says in private or public.
No, this is a huge bombshell. This is the final piece, if proven. It shows that no only did he fire Comey because he didn't like that he wouldn't play ball on the Russia investigation, but that he thought the firing would have a tangible effect on the investigation. Some people have suggested it doesn't make sense to think the investigation would go away just because Comey was fired, but this would tend to prove he at least thought it would. Plus, he told this to the Russians of all people. I can't wait for Russia to deny this and offer the transcripts again.