Not being contentious here but I would say this: When being Uke as a teacher then what are you doing?
I for one am teaching the nage how to 'throw' and indeed how to 'throw' me.
Thus any teacher worth their salt is teaching how to.
So any statement of how another couldn't shows a lack of good teaching from that viewpoint, by the uke.
I read the claim that certain people are or were unthrowable as saying simply that some people can be thrown only when they allow it, but can't be thrown against their will.