What advantage is there to getting a PhD in the life sciences when you can do research with only an MD?
The trick, actually, is to get that job. Without a PhD or other demonstrated research experience it might be harder most places. Most MD programs are focused more on patient outcomes than on science per se. 

But yes, if you can convince someone to hire you then you can probably learn research methodology along the way. This will include a lot of things, including something of statistics and more on lab process than is probably normal in the MD program itself. Publishing in the scientific literature is also something to be learned along the way and isn't especially typical in the training of an MD. 

The advantage of the PhD on top of an MD is that the focus is precisely on the scientific (rather than clinical) aspects of medicine. You learn to do science under the direction of scientists who also have some expertise in biology and other aspects of the wider medical area. 

Note, of course, that Dr Fauci is 79 years old. He has had a lot of time to learn the craft and has been positioned in places where it is well practiced. 

Note that in some places, an MD is, in fact, a research degree and requires an earlier degree in medicine. But this isn't the case in the US or Canada. See: https://en.wikipedia.org/wiki/Doctor_of_Medicine