This happens with human-generated executive summaries. They can omit seemingly-innocuous things, focus on certain areas, and frame numbers in ways that color the summary. It's always important to know who wrote the summary if you want to know how much heed to pay it.

This is called bias, and every human has their own. Sometimes, the executive assistant wields a lot more power in an organization than it looks at first glance.

What the author seems to be saying is that the system prompt can be used to instill bias in LLMs.