Maybe the init system in the 90's was similar when Slack was compared to contemporary Linux distros. There is very little modern resemblance. I wish this perception would go away. Spend a day with both and I'd put money on an honest person never making this claim again.
Late in reply - I switched off from Slack to FreeBSD around 2006. Perhaps things have changed, but Slackware generally followed the BSD conventions of what goes where as opposed to say RedHat.