The previous sentence from the one you quoted states that
“Tao also has two brothers, who are living in Australia. Both formerly represented the country at the International Mathematical Olympiad”
His (younger) brother, Nigel achieved two bronze medals (placing 132nd and 114th overall) at the age of 15 and 16 [0]. Quite good, specially at a younger age than most participants.
Terence [1] got a gold medal (13th place) right before completing 13 years of age. He achieved a similar feat (28th place) around his 12th birthday and a more humble performance (87th place, but still better than his brother's best) around his 11th birthday. At 11, better than 58% of his competition, some of the best 17 and 18 year olds at competitive math in the world.
He is cited as "the youngest bronze, silver, and gold medalist, respectively, in IMO history." [2]
These are entirely different levels.
[0] https://www.imo-official.org/participant_r.aspx?id=783
[1] https://www.imo-official.org/participant_r.aspx?id=1581
[2] https://en.wikipedia.org/wiki/List_of_International_Mathemat...
He was probably lucky to be the younger brother growing up.
But of course that doesn’t matter. One could equally hypothesize being the oldest allows for most attention from the parents, etc.
But then again, Judit Polgar, by far the strongest chess player among the famous 3 Polgar sisters, was the youngest…
In my opinion, this sort of “human peak” genius performance requires more than small differences in upbringing.