Ok I see, I understood the first sentence like you then but wasn't sure what was the point of a blanket statement there. I do feel like this this is something measurable with tool it could be easy to prove but I guess this isn't the point of this paper.
I doubt there is any proof that some kind of system activity is still taking place while in airplane mode, but that might be irrelevant.
For some people, depending on their threat model and personal preference, what's important is that it's impossible to prove beyond any doubt that this is _not_ the case.