Usually if you want to access this area you'd have to mod the game, use cheat engine, artificially lower the framerate with external software and clip through the door, etc. Here's an easier way.
1: Go to the position above the door as shown and pull out your board, if you're in the right spot it'll levitate you over the invisible wall.
2. Pull the camera down with the C key if needed, as walk speed and wall climbing ability are bizarrely tied to the camera angle.
3. Get on top of the tomb as shown, and save your game just to be safe.
4: Walk to the start of the tunnel, *definitely* save your game here.
5. Carefully move around until you manage to land inside the tunnel, which usually happens while sliding. I got lucky here and did it on the second try, but usually it takes more attempts and you often fall into invisible water.