When following the tutorial, Phil clicks (t) with just the last keyframe selected. If I do that, literally nothing will happen. So this had led me to minutes of investigation until I discovered in my version of Blender (4.5.1) I need to select all of the 3 keyframes and then hit (t) and select Constant. Then, I believe, the line starts to look very much like Phil's.
Yes indeed, Sergio; well spotted!
It actually was also like that in 4.2 and long before, Blender works on what you have Selected. Phil must have pressed A to Select all Keyframes, to get the Constant Interpolation working, but that part must have been edited out accidentally.