Forget it, this doesn't come into play when running iso2usb.sh, furthermore the USB device written with dd boots, so this is not an issue here anyway

I am out of ideas.

You could try following this guide for adding persistence to a live usb. It's for Kali but it should work for you with perhaps a little modification.

Edit: Or maybe not, I don't know if Slack will give the needed option in the installer menu

Edit 2: It looks like this should be possible with Slack: "The USB version is “persistent” - meaning that the OS stores your updates on the USB stick. The CD/DVD versions (and the USB stick if you configure it accordingly) operate without persistence, which means that all the changes you make to the OS are lost when you reboot. "

Have a look here

So is this a dead issue now? Is it not possible to boot the Slackware persistent version on the Lenovo E520 Thinkpad?