I would have to agree with you. If I were Catherine, I would be very careful of what I say to such a relative. It appears that he wants to milk his connections for all they're worth, and act as if he's more important than he actually is.
That's not fair towards "Uncle Gary". The journalist quotes "a friend of Goldsmith's" who blabbed and we all know this kind of friends...
Mandrake in his article then says: "Goldsmith, 46, the younger brother of Kate Middleton’s mother, Carole, would not comment when I raised the issue with him, and a spokesman for the Duchess told me he did not wish to discuss her “private relations with her family.”
So fact is that Gary Goldsmith personally did not comment. Neither did Catherine's spokesman.
So I don't think it is right to believe only because this journalist as well as others connected Catherine's interest in addiction (which is a hugh problematic topic where youngsters are concerned in the whole of Europe, BTW) with her uncle, it is not a proven fact that this is indeed the reason. Apart from using his own car (which is a bit flashy, okay) on arrival to the Royal wedding, Gary Goldsmith has behaved impeccably. My personal guess is that he does so in exchange for a future introduction of his daughter into Catherine and Pippa's circles when Tabitha is old enough (she is still in school, isn't she?).
Binge drinking is a big topic and it is an important task to bring the dangers of addiction into the youngster's minds. Especially as Catherine and William have their own personal history with falling out of nightclubs short before dawn. So now on planning to become parents it might have seemed to them that it's a worthy task to prevent youngsters from becoming alcohol addicts.