TSC in the place shown in the video. You either clip inside the pillar or fall OoB. If you clipped inside, align, backwalk off and JumpSlash to enter the unloaded Pre boss room. If you fell OoB just JumpSlash.
The idea of this part is to walk on a really small patch of land next to the boss door hallway. Let's say you are on the left side, walking too much left here makes you fall OoB. Too much right, you enter the hallway.
Start by finding your position with the elevator, align and backwalk to the bossdoor. L-target the door, sidehop left and backwalk to the rock part next to the hole. Sidehop right, backflip, sidehop left and walk a bit more left. Now you can use a Hookshot to guide yourself to the boss door. Align yourself with the room above using the first person camera and jump to the loading zone. If the jump doesn't reach, walk backwards a little bit.
There are many other methods for this trick so if this doesn't work or isn't fast enough for you, consider finding your own method.
It is possible to skip killing the four poes and go directly to Phantom Ganon with no items what so ever. Head over to the north door and move to the banister to the right of it. Now align yourself so that you backflip into the small banister. After that make your way to the front of the platform while still being inside of the bainster and you should end up grabbing the ledge while inside of the platform. Let go of the ledge and you'll land under it. From here it's possible to reach the boss doors loading zone by jumping into the abyss.
Another method for this Ledge Grab Method:
It is possible to go straight to Phantom Ganon fight as Child
By catching something in a bottle on the same frame that the cutscene where the four poes leave the main room of the dungeon, you can walk around during the cutscene. With this you can get on the elevator before it disappears. Once on the elevator, sidehop or backflip to lower it.
In the west courtyard, you can hookshot jump on the vines coming out of the well, megajump, or hoverboost to get to the upper part of the room. From here you can enter the block room. This skips the small key needed to enter the locked door to enter the block room from the main room. In some routes this also prevents you from having to climb to the top of the block room twice. NOTE: You can't skip the stalfos key below the bow room. The bow room stalfos don't spawn until you kill the two in the room below.
You can store a Ground Jump using action swap from hookshot to bomb/chu and skip having to do the maze.