Ah; I tried out this patch - I didn't notice what it was doing at first look.
There are two relevant resolutions the browser reports: the user's screen resolution, and the viewport size. The viewport size is the size the content has to display itself and the screen resolution is the resolution of your monitor.
When RFP is enabled it lies about the resolution and says it is whatever the viewport is. This means the webpage never knows what your resolution is; although it can infer that it is probably larger than the reported size, and may be the closest resolution that is larger than the reported size (if your window is maximized.) Screen resolution doesn't add that many bits of entropy to a user's fingerprint, since they're fairly standard.
Your viewport is quite unique though: it depends on your toolbars, font sizes, taskbar/dock size, and so on.
Letterboxing disguises the viewport; and lying about the viewport will lead to rendering issues I described earlier.
Lying about the screen resolution will not lead to rendering issues (at least I'm pretty sure it won't) but I'm also not sure what the benefit is to lie about it in the manner of this patch vs what we do currently (after all, we disclose the viewport size). This patch actually leaks more information than we do currently: regardless of what my window size is, this will disclose if my monitor is in portrait or landscape mode.