It is more difficult to write simply, directly, and effectively than to employ flowery but vague expressions that only obscure ones meaning.

請問老師這句中的one指的是某個人嗎？謝謝你!

It is more difficult to write simply, directly, and effectively than to employ flowery but vague expressions that only obscure ones meaning.

是的。one 指代“人” （指作者）

that only obscure ones meaning 只能使一個人（作者）的意思模糊。