The original publication: A bizarre, humped Carcharodontosauria (Theropoda) from the Lower Cretaceous of Spain Francisco Ortega1, Fernando Escaso1,2 & Jose´ L. Sanz2 9 September 2010
In the publication the bumps on it's forearm are interpreted as quillknobs and compared to those of an extant turkey vulture. They are pretty much identical. If this was an isolated case then I would maybe have been more skeptical. But since we now know that "proto feathers" and "dinofuzz" goes back at least to the base of Coelurosaurs. It's not unreasonable to think it goes back even further. So I don't see a good reason to doubt that these bumps are anything but actual quillknobs.
I don't doubt that these bumps could be quill knobs, it's just that recently I've heard many rumours across the net of them being attachments for muscle fibers. However, as the quills are present even in ornitischians, I do believe Concavenator probably had some.
''Since the popular arm quills turned out to be just muscle fiber, there is no actual reason to depict this animal with quills or dinofuzz all along the body (although its still a possibility)''. - Durbed, dA
''..and the supposed ulnar quill knobs being part of an intermuscular line...'' - Theropod Database Blog
I don't know where they're getting their information from, but it doesn't seem to be just rumors.
Other related theropods have also been found to have scaly skin, and Concavenator is not a coelurosaur. Concavenator probably had protofeathers like its relatives, not neornithe-like feathers identhical to a turkey vulture's.