One of the things "Request desktop site" is supposed to do is report a larger effective viewport. For a responsive site, that should switch it to a desktop view. Does iOS Safari not do that?
Good question. Now that you mention it, I do not know if the problem is on iOS Safari or the website. EDIT: I just tried it on HN (which is usable enough for me as-is, I’m just using it as a test), and it seems my effective viewport remained unchanged.