I remember this happened before several years ago in a previous redesign where the top menu would just stretch to fill the entire screen. It was later changed to one with a proper max width. I guess they forgot that lesson.
I hate that it centers it with giant gutters wasting over half the width of a 4K monitor, should I choose to maximize a browser window. I'd much rather see everything shifted to the left. I'd even be OK with some soft margin still causing regular README text to wrap at a typical width.
But, if there are long code or raw text lines, or any embedded image or markdown table or other structure that is inherently wide, I want it to overflow and use that extra screen space, not get clipped into this ridiculously narrow bowling lane. That's my instinctual desire and the only reason I would expand the browser to full screen, and it is an utter disappointment to try that and be told, "no space for you."
You really need CSS that accounts for resolution and pixel density these days.