Because the author is artifically shrinking the scope of one thing (prompt engineering) to make its replacement look better (context engineering).

Never mind that prompt engineering goes back to pure LLMs before ChatGPT was released (i.e. before the conversation paradigm was even the dominant one for LLMs), and includes anything from few-shot prompting (including question-answer pairs), providing tool definitions and examples, retrieval augmented generation, and conversation history manipulation. In academic writing, LLMs are often defined as a distribution P(y|x) where X is not infrequently referred to as the prompt. In other words, anything that comes before the output is considered the prompt.

But if you narrow the definition of "prompt" down to "user instruction", then you get to ignore all the work that's come before and talk up the new thing.