Intending to support it but never testing it would be a surprise. If I were designing laptop hardware I'd probably spend a lot of time booting it from external media as part of the design process. It would accidentally be the most tested feature of the hardware aside from maybe turning it on.

You assume it's a bug and not as designed.

If they didn't want people running their own stuff on it, they would have locked it down like an iPad. The fact that they didn't means they wanted it to be possible.

Maybe or maybe not. It depends a lot on how the chip's boot ROM is designed.