1. "Semantic web" refers to data being linked and having meaning. You're referring to semantic HTML.
2. I don't think <strong> meant <em> but more so - as I understood it, <strong> meant text that had to stand out from the rest, whereas <em> meant that text was emphasised. Though I might be confusing it now with the retro-actively applied "semantics" of <b> and <i>.
Totally agree with the overall point btw, and I do tend to use them in the way you described.
I guess it was an HTML 4 thing, the standard at the time of Markdown, https://www.w3.org/TR/html401/struct/text.html#h-9.2.1
<em>: emphasis.
<strong>: stronger emphasis.
I see that HTML 5 has tweaked their descriptions, https://html.spec.whatwg.org/multipage/text-level-semantics....
<em>: stress emphasis
<strong>: strong importance, seriousness, or urgency
Anyway, it was all a whole lot of "semantics-washing" anyway; it's lot like people strictly adhere to strong rules of when text should appear bold or in italics anyway.