Slackware's pkgtools depend on bash and GNU coreutils. As a result, it's impossible to replace bash+coreutils with busybox, toybox, ubase+sbase, ...
Kiss linux has been created with focus on highly portability and user choice. There's no lock-in. Users can replace bash with dash or GNU coreutils with sbase+ubase and vice versa.
Slackware isn't alternative to Kiss linux at all. They have different goals and mindset behind them.