As somebody pointed out earlier, he probably had no clue what he was doing there really and can't do off the cuff comments. At that point he was on the outs with the Firm I think, so he was probably refusing help because his lil D list actress told him he is a God and can do things his way.
Note how he gaslights Rhiannon by saying "Don't do this Rhiannon!" thereby blaming her simple question for his lack of knowledge or manners.
Nope, sweetie pie, I have to disagree on this.
Her question was:
"That short conversation, what do you hope to achieve with it?"
And he answers: "What? Ask them?" Pointing behind himself.
She clearly called him out on what was reported about their African tour a couple of times, only showing up for ten minutes to have the photos taken and off they went. They didn't bother with the organizations or people, didn't stay long enough to learn about where they went.
So asking him what he wanted to achieve with his short(!!!) visit was calling him out on his/their strange and suspicious behaviour.
I do think everyone in the royal press corps already knew they were just trying to get global coverage before jumping ship and her question made this obvious to him. Thus the "don't behave like this, Rhiannon!". He feared exposure of their plans and shut her off immediately.