Yes, unfortunately I think it would be very unlikely for two such different manufacturers to assign the same signals to the same DIN socket pinout. I didn't have much luck finding the sega DIN socket pinout, but here is some info about the C128 video output here:
http://www.softwolves.pp.se/cbm/mask.../scart.en.html
It seems to suggest you need to be using the DE9 output (if you have one) for output to SCART.