You can argue that any limitation is simply due to the feature being mislabeled, but in this case it seems pretty clear what people both expect this feature to do as currently named, and that happens to match what people also seem to want, and that would include changing the viewport to make certain you actually get the desktop site functionality.
Request mobile site is really only applicable to the small amount of websites that serve a m.domain.com.
Now, they could have an option to "request desktop SIZE" that would result in sites that are responsively built rendering in "desktop size" mode.