As he is executing his upward haul he begins his forward cast. I don't see how the rod will begin loading with his line hand moving upward.
You are right. The rod won't start loading until it begins rotating. Lasse is using the technique that Joan Wulff described as " slide loading" which is actually not a very descriptive term to use since the rod cannot load when it is parallel to the line.
What it is really is a re-positioning move to get the body, casting arm and hand into position for the rod rotation. His body, arm and rod are gaining momentum while the loop is un-rolling behind him and as his hands come together he starts his rod rotation and the start of the haul.
His hauling arm isn't long enough to reach his casting hand when it is so far behind him. I think nearly everyone casting long distances does this. Some use an even more weird technique described by George Roberts.
And that technique is to be still shooting line into the backcast while moving forward as Lass is doing. Then clamping the line at close to the time of the start of rotation.
I did not think that technique was even possible when reading about it. Then I filmed myself casting with the light, and discovered that I have been casting exactly that way, as Roberts described, for probably 40 years.
I do not see the slack you described after looking at the video again several times though.