Its not really a cure - its more a first case where a common preventative measure was shown to work.
Newborns are not infected with HIV in utero - this is exceedingly rare and universally fatal to the baby. Rather, they are infected during birth, when damage ti the placenta allows maternal blood to cross over to the foetus.
What the researchers did was give this child a massive dose of anti-HIV drugs immediately after birth (and continued them for ~6mo). This is not unusual for babies born to HIV positive mothers - what is unique is the MD's did a sensitive test for the HIV virus and found that it had been passed onto the child. Whether this is a 'cure', or simply a case of the anti-viral preventing the first round of infection, is unclear. It is also unlikely this will be a cure for most people, as it requires drug administration within hours of infection.