Disclaimer, I'm a dev at Rackspace and helped run 2015's instance of the internal conference example linked in OP.
Speaking only from my own experience, this kind of internal dev conference can be especially useful for large companies (like Rackspace) that have teams of developers spread across the world, who may never otherwise work side-by-side. You mentioned management buy-in: a key selling point is that even outside of the talks+workshops, an internal con provides a discrete, intentional space for these spread-out teams to work together for a few days, outside of the pipeline of their "regular" work. Essentially you are bringing the benefit of hackathons & sprints to internal projects, using the structure of a conference for framing.
As far as "how to get devs to speak" -- lots of devs do like speaking (or teaching workshops), and maybe it's easier to do that for an audience of immediate peers and coworkers? But for many public speaking IS hard, and a way to mitigate that barrier is to consider the idea of an unconference [1]. When you move away from the standard tech talk format (stage, mic, podium, etc) and towards an "unconference session" (white board, circle of chairs, participant-set agenda), you can get a surprisingly high level of participation.
I'd be interested to hear of any experiences with this kind of thing at a smaller company, but essentially I think the core advantages (sprints, intentional technical discussion, knowledge exchange/training) are things that can be scaled up or down at will.
1) e.g., http://transparencycamp.org/about/tips/
