Author here, I don't necessarily think they need an in-house kernel dev, but it wouldn't hurt. Mainly they need someone familiar with the ecosystem that knows how upstream works, knows how to interact with distros, knows how to do some kernel debugging themselves, etc. Hiring a kernel dev makes sense because they'd already have those skills and can help out on the firmware side for Framework as well.

Realistically it looks to me like they need a reasonably technical desktop Linux user with social skills and anything else is a bonus, but I have no idea what skills they have internally