Why is it correct to say that decomposers are also consumers?

The term "final consumer" is more commonly used in marketing.

In terms of ecology, decomposers live by constantly breaking down and absorbing organic material which can be alive or dead, which is done on a molecular level. An example of this is fungus on trees.

You may have the Word "detritivore" in mind, which can break down feces, animal flesh, and other organic matter. Although they are not specifically referred to as final consumers, they do consume consumers at the top of the food chain after their death. Since they are the last consumers before the organic matter is reduced to soil, they can be considered "final" consumers.