I am guessing this message was sent to let people know in advance that she will be doing a public outing (and by knowing in advance is hoping there won't be such a big thing made out of it thereby taking away from Charles's Day as he will not want that either?). Although another part of me wonders if Kate being there is, in part, to distract from wondering how Charles's treatment is going.
And perhaps also a tiny bit of hope that people won't scrutinise her appearance to every minute detail (which is exactly what will happen).
Its strategic no doubt but it strikes a nice balance between personal and professional.