Slackware uses the sysvinit program, but doesn’t have System V-style scripts. Which is somewhat confusing, but sysvinit is a basic init program that will just do whatever /etc/inittab tells it, so you can write your startup scripts to work however you want.
Slackware uses what people tend to call a BSD-style init, but it’s nothing like the modern BSDs, nor the older BSDs, not really. If you use Slackware, you’ll learn how Slackware’s init system works, but that’s about it.
No, you’re right that it has scripts, they’re just not the scripts used by SysV-style init systems. They have different names, are in different locations, and are executed differently.
I used Slackware for several years back in the 90s, and from that experience I’d recommend against learning it. I mean, with VMs today it’s simple to try new distributions, so go for it, but I’d put it waaaaay down the list of distributions/operating systems to try. If you have anything else you’re interested, put it first. Slackware is standard Linux so there’s nothing really special you’d find when using it, and it’s just a painful experience in general. I think some people will argue that it helps you “really learn Linux”, but I don’t think so. It just helps you learn Slackware’s idiosyncrasies, and learning pretty much any other distribution would be more beneficial than that.
Slackware has advanced from when I used it in the 90s, but only barely (they have a network-based package manager now, I guess, although it proudly avoids dependency resolution!)