I'm telling you, .. it's just a different playstyle.
He starts to get it toward the end. You don't focus so much on landing basic attacks, other than for farming. You play around your abilities like a mage.
He also mentions how his abilities deal magic damage, which is not true. The only one that deals magic damage is Gravitum's Q. That means armor penetration is still a great option to build into.
This did highlight for me another good combination. Since Severum's Q doesn't really have any damage scaling with AP, you aren't able to effectively use it as a sustain weapon. However, using the Q from Crescendum with Severum in your off-hand applies damage based on 50% of your AP for every attack for the duration, and heals you based on that damage.
Also, I did this testing myself in the practice tool and you can see it in the video. When you have Lich Bane (or any Sheen item), the Spellblade passive is applied by through the off-hand attack. When using Calibrum, right-clicking a marked target will apply the Spellblade passive on the resulting attack, but when using Severum and Infernum it is applied automatically. Crescendum and Gravitum require an additional basic attack after using the ability / placing the turret.