Just as a note, the monitor is an early 2000 one, and the image shown is in defaults as the monitor syncs to it. I didn’t manually adjust it but it is indeed in 640×480.
Regarding what PAR developers used, the probably sticked with NTSC. Or at least only with one. Having all the resources or data calculated for both would be too much work, but possible. Stretching in frame buffer would not be likely in the Dreamcast though.