[mrsteveman1]

There was a time when it looked like they might switch their Airport and Time Capsule routers, which previously had been running VxWorks and NetBSD, to a variant of iOS. But then they shelved the entire product lines a few years later.

It did turn out to be true that Apple was moving to run iOS (and later on, even APFS) all the way down to watch hardware, and later on the HomePod which is the functional successor to the audio features of the Airport Express.

Had they continued producing routers, it's all but guaranteed that they would be running firmware similar to the HomePod at this point.

The reason they would be building XNU for the bcm2837 however is probably much more "boring", like allowing various teams inside Apple to test some low level change being planned for the kernel, perhaps in the network stack, without the added hassle and potential for leaks that would result from using some other unreleased Apple platform to do it.