Yeah agreed (what in vitro means to different people is a whole other conversation :-P), but that sentence makes it pretty clear that by in vitro they mean not in cells (because it says, "and not inside cells"). Agreed, of course, that this doesn't necessarily mean it happens in cells in an organism (though in the authors' defense they do examine three different cell lines).

Edit: For clarity - all the in cell work is cultured cell lines and not cells taken from an animal model or in situ imaging.

Literally my only point was that 'this might not matter for biology because we've not even seen it in cells' is no longer true. It still might not matter for biology though!