Yes, SUSE indeed started as a modified version of Slackware. For a long time, SUSE's package categories names were based on Slackware's disk sets names (even after rebasing away from Slackware). Later, SUSE hired the author of the Jurix distribution and rebased on top of that. SUSE and Jurix were both from Germany.
History of Jurix: http://linux2.mathematik.tu-darmstadt.de/pub/linux/distribut...
SUSE had a stronghold in the nineties in Europe. E.g. here in The Netherlands, many bookshops would cary SUSE Linux boxes.