You have to start from the general proposition that frequent flyer miles cannot be transferred from one scheme to another. I think that the surprise that people are expressing is that someone can have accumulated a substantial number of miles in such schemes without knowing this.

Sometimes there are exceptions, such as for Avios where the three airlines and one additional rewards scheme within the IAG group are allowing transfers between them.

Between some schemes, there are also some ways of "laundering" miles through a third party (eg points.com). But (as has been said) this usually involves someone taking a huge chunk out of your miles for the privilege of doing it for you.