The impression is that a "DevRel" spends all their time writing talks, traveling to conferences, pitching products and speaking. This makes sense, because the "DevRels" you probably see the most are the ones that are doing these things, and you don't see them doing anything else. A lot of people are surprised to discover that traveling and speaking is actually a very small part of my job, given the number of talks I give, and how often they see me at events.
I think part of this perception comes from the conflation between an advocate and an evangelist. There is endless debate about what these terms mean in the context of developer relations, but for me, the difference is this:
* Evangelists want users to know that a product exists: They're speaking to developers, about the product. They probably report to & work with Sales/Marketing.
* Advocates want products to know that their users exist. They're speaking to product owners, about the developers. They probably report to & work with Engineering/Product.
Both roles are equally important (and sometimes done by the same person), and the "publicly-validated credibility" the author talks about is just as valuable to the product team as it is to the sales team.