It's a habit of the community. Tao actually elaborates on what he means by this in the same paragraph. I'd like to offer a charitable interpretation. By Tao la

It's a habit of the community. Tao actually elaborates on what he means by this in the same paragraph. I'd like to offer a charitable interpretation. By Tao labelling it as "remarkably elementary", he encourages less experienced mathematicians (including students) to go and read the proof for themselves. He's advertising a low barrier to entry for certain parts of his audience. This isn't always the case with whatever he reports on, so it's worth pointing out when it's true.