How do I remove a separated object? I am trying to follow along with this tutorial. I had to put its speed to 0.25 to be able to view it.
In this tutorial it is shown how a cube is separated by selection, and then some part is deleted.
I have taken some screenshots:

I could replicate the first 4 steps, but not step 5.
I have added 2 arrows showing the problem. I don't understand how the author of this video deleted the rear geometry of the cube.
In step 3, he separates the front from the back.
In step 4, he has the entire cube selected in Object mode, and in step 5, the rear geometry is gone.
How did he do that? How did he select the rear geometry to delete it?
Here is a video of my own attempt.
https://www.youtube.com/watch?v=gNhwZ0L9QIQ
Edit: I have just noticed that in the end of the video, I actually succeed. It was due to help of the answerer to my post, Timaroberts.
Thank you!
After separating the geometry it is it's own object thus, I presume that either the tutor deleted X or hid H the  rear geometry once returned to object mode.