Changing the resolution on a CRT normally doesn’t make the picture smaller. There is no native resolution, phosphors are not pixels. My Viewsonic would display 640x480 or 1600x1200 on the whole 21” regardless. You can also watch the video, it’s not using a smaller area.
I believe the limitation is bandwidth, not the electron beam.
There is a limit on the spacing of the colour bands though. If you want colours then you have to hit the spots where the correct phosphors are and this limits the usable resolution.
Yeah I didn't think it would make the "pixels" smaller, but the beam would need to pulse less often and therefore could travel more. Maybe I'm misunderstanding what they did.
Edit: I guess when you think about it, someone born then would be entering high school now. Fuck I'm old. It's weird because all the 2000s just blend together for me, there is nothing defining the decades anymore, like 80s = cocaine and big hair, and 90s = neon and "radicool" stuff.
I loved my 1600x1200 Viewsonic, used it till 2010 or so. The flicker wasn't ideal, but man the colors were so much more vibrant than shitty LCD screens of the 2000s were capable of. These days, I think Apple's fancy LCDs with HDR win on all fronts, but it took a while to get here.