Why not? See that logo in the upper-right of the page? That's the UN logo. Know who elects its members? ECOSOC, another UN body. Know who nominates at least five of them? The WHO, yet another UN body.

A commission of ECOSOC is also responsible for the scheduling of drugs under the same treaty that set up the INCB.

"Independent" agencies are not unusual in governmental contexts. The FCC, FDA, and FTC are all prominent US examples. "Independent" doesn't mean they're not part of the government, they're just outside the direct control of other parts of the government.

Every time, for example, the FDA bans an import of <whatever>, headlines spring up both inside and outside the US about "the US" banning <whatever>, generally followed within the article by explicit explanation that it's the result of an FDA action. It is not unfair nor even misleading, the FDA is part of the US government, and on the world stage, the US is answerable for its actions.

That is exactly the wording The Guardian used, although maybe we shouldn't take everything they write at face value.