Not sure what the graphics controller was capable of, but the 'paperwhite' monochrome monitor which was offered then supported 72Hz refresh rate. That was a lot then and most people won't notice the remaining flicker, but it's not 100Hz. Op might be misremembering.

(I'm a bit sensitive -- around that time, I removed a connection on the MB of my Schneider, nee Amstrad, CPC 464 so that it would think it's the NTSC version and would output the screen's content @60Hz and adjusted my monitor accordingly. 60Hz was just about bearable, 50Hz (PAL) only with dark background)