That used to be the advice, but the W3C eventually agreed with you. The spec describes <b> as "a span of text to which attention is being drawn" (https://html.spec.whatwg.org/multipage/text-level-semantics....) and no longer mentions boldface as what the tag "means", it's just how it's usually styled by default & convention.
Huh... the documentation there for i is actually even more interesting to me: it seems like I'm actually actively "not supposed to be" (at least anymore, if ever) using em for almost every reason I've ever used it?!