I had to comment on this thread because I am literally the female version of the guy you are talking about (extremely introverted) and I can tell you right now why he sent you the message he did.
I'm sure he likes you as a person and is being genuine in the moments that you spend together. He probably finds you very attractive, interesting and enjoys chatting and laughing with you... but introverts tend to know themselves very well and he would have realised pretty quickly that he would only be able to be around you in small doses.
You sound lovely. But you describe yourself as being extroverted. I imagine you are engaging, bubbly and a very vibrant personality that attracts men easily.
The problem with extreme introverts, is that we find being socially 'on' very tiring. While I enjoy being around extroverted people (vibrant, bubby, high energy, etc), if I am exposed to it for too long I need to lie on the floor because I get too tired! (I am not even joking)
People like your lovely introverted colleague, need time out by themselves to recharge after situations like this. My own personal ratio is 1:4. For every 1 hour of being socially 'on', I need 4 hours to recover.
I am very well aware that a relationship between myself and a very extroverted person is doomed to fail, no matter how much I like them, because I would get too drained and eventually resent them. It sounds like your lovely man recognised this very early on, did all the thinking (as we introverts tend to do) and told you very honestly how things were. There's no getting around this without one or the other of you having a personality transplant.
There are no mixed signals here. He does like you, he is being genuine. He enjoys laughing and chatting with you. He probably avoids your eyes because he doesn't know what to do or say (awkward). I'm sure he realises you're confused.
He was being completely honest when he said you were too full on for him. Nothing against you as a person, you are just not compatible.
I hope this helps you OP. :)