Thanks for reaching out. I was an MDN developer, but I think the opinions of the writers are more accurate here. This document is focused on learners, and it takes skill and empathy to write for that audience. I’m pretty far along in my developer journey (I’m likely to go to Wikipedia or read the RFC directly), and I don’t trust my instincts for learner-level documentation.
But, you asked for my opinion…
My opinion is that the paragraph is OK (200 OK even ), and it is close enough to how things actually work. Yes, in most cases, the HTML is in that initial request as well, probably spread over some TCP/IP packets. However, the information on this page, plus that in the following paragraphs (talking about requests for follow-on resources like CSS and JS) is enough for someone new to this information, and will give them the mental framework to start using developer tools and dig in further. The developer tools should be the learner’s next step, not network theory.
I think talking about the OSI model will be confusing at this stage. I think the OSI model is most appropriate if you need to work lower in the stack, such as setting up firewall rules, or are transferring knowledge about one protocol to another, such as learning HTTP 1.0 vs 1.1 vs HTTP2. I don’t think it adds much, other than confusion, at this stage of the learner’s development.
@soroushbakhtiary you are correct that the HTTP message usually includes the status line, other HTTP headers, and then the content, all in one message. There are exceptions - if you or the browser makes a HEAD request, the server is expected to only return the status line and headers. And, if the message is big enough, like a video stream, the server may require serving via byte ranges, so multiple HTTP requests are needed to fetch all the content. But I’d expect all of the HTML content returned in the first request for 99% of websites. HTTP/2 had a feature where you could also push some of the JS and CSS in that initial response payload, but I think that proved too tricky to implement for most web servers.
When you are ready to directly talk to the MDN content team, see the https://github.com/mdn/content/ repository. There are some contribution guidelines, as well as a few ways to contribute. This question could be an issue in that issue tracker, or you may feel confident enough to jump to a pull request against the page content.