AFAIK the interaction between Dream Coil and Eul's doesn't work the way author of the video thinks it works.
If you look closely, you'll notice that cyclone (Eul's active) not only changes your z-axis position, but also rotates your hero, and more, it rotates your hero not around the center of your hero, but around a point that is close to center, resulting in «wiggling» of your hero. So what actually happens is, you walk to an edge of Dream Coil AoE, use cyclone on yourself, and once your hero is lifted up, it wiggles back and forth for a small distance (on xy plane, not on a z-axis). And when that small xy movement causes the distance between you hero and a Dream Coil to become too big, dream coil leash snaps, but doesn't stun you because your hero is invulnerable during cyclone.
Also, in the beginning of the video he compares toss with eul's and asks «why seemingly similar spells work differently». Well, it's because they are not similar at all. While Toss moves you upwards leaving nothing under your hero, when you are cycloned, there is a (you guess it) cyclone directly under your hero, and cyclone itself is a kind of a «unit». While you can't select, attack, or really do anything with it as a unit (similar to DW's shadow realm), this unit still has a collision size, so unless you are phased, you will collide with the unit and be «blocked» by it.
Quite some time ago cyclone had no collision size (or maybe it had a phasing property) and units were allowed to pass through it, which was abused by SFs to deal maximum damage using his ulti.