Yeah it does explain it if you use your brain for like half a second. Is the orange portal moving down to the cube or is the cube moving up towards the orange portal? We have the reference frame of the cube so it looks like the portal is approaching the cube.

What you also conveniently forgot to mention is his thought experiment also explains relativity where you can't tell whether it's acceleration due to the elevator moving up, or gravity pressing you down in the elevator while it's stationary.

Since you can't tell the difference between the two scenarios the cube should only do one thing in both scenarios to be consistent with relativity. If it doesn't it breaks it...