Why the United States is described as divided, not declining

Do failures such as badly handled shipboard fires prove that America is a declining power?

No; it is described as a divided power whose internal focus limits momentum without yet amounting to a sustained loss of capability.

One country's rise does not automatically prove another's decline. A stagnant power holds roughly steady, a declining power loses capability and status, and a divided power spends so much effort on internal conflict that it struggles to build momentum. The United States fits the third category because domestic struggles consume attention without necessarily erasing underlying capacity. Rising dictatorships can appear strong because coercion produces visible unity around an external goal. Democracies look messier, but the Second World War shows how much latent energy they can mobilize once a shared purpose emerges.