Yeah, that's bullshit. Open any CS book on your shelf and you'll see different fonts, different sizes, different weights, different background colors, etc. Those books are not written for kids, they're written for adults—for professionals. The goal of a writer is to control the reading process; changing what the text looks like is one way to do that.
We separate the text into paragraphs to delineate different ideas, but also to space out the text and give the reader a chance to breathe. We organize points into bullet points because they're easy to spot, easy to scan, and we don't need to make the text longer by adding flow between each point. We sometimes use frames and a darker background for summaries, auxiliary information, etc. to show that it's not part of the main text. We use large letters to identify different chapters and sections. We use a monospaced font for code examples. We write technical terms in italics when we first introduce them and want to highlight that fact to the reader.
There are a multitude of ways in which we format text so that readers can better follow. Bold is just one extra tool in that arsenal. Don't abuse it, but don't forbid it entirely either.