This is a phrase that I think many of us have said to or about ourselves and I think I have had an epiphany about it, much in the same way I had one about 'I can't art'.
First what is teaching? What is the first thing that pops into your head? School? Classroom? Lectures? PowerPoint presentations?
What about, mentoring? Long talks? Geeking out? Sharing a link? An article? or a Technique? A recipe? A book? Resources? Do you think of any of that.
What is teaching is as wide of a subject as subjects there are to teach and teachers to teach them.
I can't teach to me was... I am nervous in big groups when the focus is all on me for a prolonged period of time. It's I can't teach someone how to do things my way because I just do it!
What *I* CAN do is... teach someone how they can do it. Help them discover what their way of cooking, organizing, research, redaction, and all that goes with it is. Help draw out what works for them while sharing the things that may or may not have worked for me. I can direct someone to better resources. I can be an ear for their theories and suppositions and help them work out the logic. I can teach, just not in the way that others may see as 'teaching'.
Teaching is not black and white. It is not replication of what another does. Teaching is a form of sharing and we as teachers need to modify to how a student learns as well as to what our own style of teaching is.