For avionics, I doubt that they would use a traditional os. As far as I'm aware, Microsoft doesn't have safety-certified builds of Windows with a real time kernel. Certifying a Linux build would also be a huge and costly endeavor. What they are likely using is a certified RTOS, like Vxworks, RTEMS, ThreadX, SafeRTOS, etc., or even Ada with the Ravenscar profile. You don't really "develop" applications for these, you instead incorporate them as libraries inside your application and compile the RTOS into your application, and then run it on bare metal. Infotainment systems on the other hand will use more traditional OSes.

A lot of the presentation seems to be rather typical of the aerospace industry, which is all about safety. Im not too convinced that this is due to Boeing being Boeing, but rather that DO-178 compliance is a bitch, ITAR can be another bitch, and certifying not only a single build of the Linux kernel but also an entire distro build will be a superhuman effort. At best it'll take a long time with a sizeable team. Not sure that would Boeing be filling to fund that.