Hello, @Elise, and welcome!
Thank you for asking. I have been experimenting with software-defined networks, from simulated, to emulated, to virtual, to mostly real for over ten years; I will say that's not a dumb question at all, because SDN can be defined differently or mean different things. The short answer is, yes, SDNs do exist (a little) and, yes, we have built a global cellular SDN! Woot!
What we mean by and our philosophy concerning SDN is, basically, building a programmable network. Much like software on a general-purpose computing device (like a PC) allows the device to perform specific tasks and even to be repurposed, an SDN allows the network to be specialized, customized, and to be made more intelligent. Similar to hardware-assisted virtualization (an entirely different, although somewhat analogous, topic), we are able to combine cellular hardware frontends, special networking hardware acceleration and security, and software to build an SDN capable of massive scale, as well as strong security and isolation. Essentially, we can leverage the unique advantages of both hardware and software, and some secret sauce layered in there as well, to create a more advanced and intelligent network that is also adaptable to the needs of different use cases.
What this looks like specifically will be coming out as we roll out new features, but expect security enhancements that properly fit specific applications' needs, agility, and more advanced application intelligence to start. What this will become is a transformative movement towards better cellular network support of, assistance to, and cooperation with existing/emerging IoT application needs by adapting the network to be like a well-fitting glove to individual applications, instead of being funky and nuanced like cellular has been in the past.
Looking forward to building the constantly-evolving cellular network of the future!