I really don't understand what's wrong here
I followed this tutorial from Blender itself and every node setup that I built followed that Blender make, but I get different result compare to it.
Here is the Blender Version result:
And here is mine:
I make mine using Blender 4.1 (I don't know what version did Blender use to make their files)
Also Here is my file and Blender's
(Blender) : 
(My File) : 
Before solving the problem, you should reduce the size of the cloud curve - yours is 6010 m wide and 3352 m high, the original is just ca. 71 m × 40 m. As soon as you would get this volumetrics to work your PC might crash because of the millions of voxels.
So, first of all I scale it down to 1% of its size with S 0.01 Return. Then, very important, apply the scale with Ctrl+A > Apply > Scale.
Now the important thing is, the Geometry Nodes nodetree works with the Bézier curve lying in the (local) XY plane, therefore in the original file the object is rotated 90° on X to view it from the front.
You have not rotated your object and the curve is lying in the XZ plane. Here is the difference, the curves without GN enabled in comaprison:
So what you have to do is go into Edit Mode, select all with A and rotate the curve by -90° on X with R -90 Return. If you then rotate in object mode by +90° to again see the curve from the front view, you will have this result:
To quote the Blender manual on this matter:
"The Fill Curve node generates a mesh using the constrained Delaunay triangulation algorithm with the curves as boundaries. The mesh is only generated flat with a local Z of 0."
