I did not run the tool yet. But the problem is actually that my external monitor only turns on after GNOME has fully started. And since logind starts before GNOME, it thinks there is no monitor and suspends.
Creating this file works around my issue: /etc/systemd/logind.conf.d/dont-suspend-with-lid-closed.conf:
[Login]
HandleLidSwitchExternalPower=ignore
The underlying problem here is that by default Grub and Linux only post to the same display to which the BIOS posts. So the real solution would be to implement BIOS on External Monitor