They don't have to post all the data, just a snippet like several usernames and their passwords, anything pretty much. Also, think about it, anyone could write up an email like this, hell, I could if I wanted (not that I do). It's really easy with some basic technical knowledge to write up some fake email claiming to have hacked a site and ask money for it.
Yes, I did get the point of why it would make sense to publish at least something to prove the claims made.
What I'm saying is that doing so is not a de facto standard of the hacking industry, and that the lack of it does not exactly prove anything either.
It is of course very easy to just send an e-mail like that, hoping that at least someone would pay the price.
And still, that possibility does not mean that the message in question would be fake.
What he probably meant to say is that he doubts your experience in IT security due to the fact that you believe a baseless email with no proof. Sadly, I have to agree.
Your analysis matches with my guesswork as well.
However, I'd like to point out that I'm not "believing" anything, which can be clearly seen in my original post. I'm making the educated guess, based on the 15+ experience I have from the field, that the message is more probably real than fake. Even this estimate does not mean I would just blindly digest my initial analysis and start considering it a proven fact. It's an estimate, made by someone with experience, and questioned by someone of whose expertise in this area I'm not yet aware of.
You don't have to be sad for what your estimate of me or my experience is.
If I were you, I think I'd still change my password - can't loose much doing that anyways.
-j.