If they’re using their monopoly power to disadvantage a competitor while leveraging that disadvantage to compete against Facebook, it is definitely anticompetitive and could get them sued by the government and Facebook. It does conflict with them ramping up their own ad business if it’s anticompetitive.
Giving users privacy is indeed a good thing, but Apple is not an ethical company so I’m not sure why they should get the benefit of the doubt. Without inside information into the reasoning, it is wrong to assume they are doing this for peoples privacy. They could just as easily be doing it for selfish reasons.