Do not confuse "regular CRT TV" with CRT computer monitor!
A bare CRT would not have a native resolution (up to its video bandwidth). But when you put control electronics and an interface (Composite video, component, or VGA) around it, then you've defined its performance.
S-video is a variant of the NTSC or composite video. That's 525 interlaced lines, about 4 MHz bandwidth, and the luminance signal is separated from the chroma signal.
An "ordinary" 4:3 TV might display about 300 x 480i resolution.
A "good" TV might display about 400 x 480i resolution.
You need a "really good" TV or a computer monitor and VGA to get to 640 x 480i.