1h sword + shield still counts as single wield. Just doesn't work as dual wield setup. This sword is pretty weak to balance things out. If you put very good dps as main hand and +1 stormblast as offhand just to get double stormblast, thats what was op, and doesn't work anymore.
As for your question, you mean specifically for samael with endgame items, or for untwinked run this guide is about?
For crazy gear memes dual wield is better, lot of those showscase videos use absolutely insane gear, umo projects, you can get more stats and damage off 1 item than whole gear this guide features. These is next level stuff, and two weapons are superior.
When it comes to untwinked run, here is some of my thoughts:viewtopic.php?p=355987#p355987
Sword + shield is very viable after numerous little buffs to sword, shield helps great deal too. It is very competitive, much more than initially when this guide came out in 1.0, or even earlier before sigma where deathgaze absolutely stomped iron spiral.
I may even create alternate tree and gear section for sword+shield variant because it is just good.