Even highly acclaimed research institutions and journals unfortunately care mainly about money and exposure. Sinclair undoubtedly creates research papers of high "impact" - in the sense that they are sensational and oft-cited. It's just that no one else can reproduce his findings and that the actual clinical impact is unimpressive. Moreover, while appearing reasonable at a glance, many of his statements fail under scrutiny - the text I linked by Brenner really does quite a good job in this regard.

There is (unfortunately) not a culture of actively trying to disprove the research of others in today's scientific research, even though it is in principle one of the pillars of science. The fact that Sinclair's claims are so over the top that they merit a response like this truly is special.

Being a professor certainly seems respectable from the outside, but the universities are full of them nonetheless, and among peers the title is worth less.

He has built a career on sensational (irreproducible) findings, overinterpretation and exaggeration.

Here's your link for proof. I know of few researchers who have been called out in such a fashion.

Cell is certainly quite prestigious, but as I'm sure you are aware from your own field, the prestigious journals are more interested in sensational findings than in scientific rigor and reproducibility. Moreover, the peer review will depend on the specific reviewers and there's quite a bit of backclapping going on in this particular field of research.

That said, I cannot speak to this particular study, as I have not yet read it myself.

He's definitely not well-respected. He has a following of biohackers, but many researchers consider him a snake oil salesman.