I was asked about this recently. And I have debated it a little at work. So I thought I should get it down. A technical evangelist was called technical marketing in the past. But it is in fact more than just technical marketing. You need to be able to talk about your product, and in depth, but also, you need to talk about what is around it. I have helped customers be successful with a variety of different backup tools. I even led implementation projects in the past for TSM and Backup Exec. I use that as examples of the background info that is needed to be a technical evangelist in my space. So I guess I am saying technical knowledge of your product, and experiences in the same space. But, and here is where I veer off a little from the common thinking. You need to be able to talk business. You need to be able to talk about how your product helps business be more successful. Another way to look at this is you need tactical skills but also strategic. The strategic is what makes an evangelist - not just the tactical of the product knowledge.
So I hope that this helps?
And yes, I think of myself as both a Technical Evangelist for Veeam, along with being a product manager too. I just did a very long trip with stops all over, and I did both Technical Evangelist work, and product manager work. It was very worthwhile for both in fact.
Let me know if you have questions or comments! BTW, this is what Wikipedia thinks.