Extra tricks seem to be required (2.81a)?
I have been following along using 2.81a. I got stuck on a couple of subtle things at "Interpolation". Eventually I blundered into some answers, but maybe worth a mention for others?
- "Interpolate" dropdown only works if cursor is between orange keyframes. (You actually moved it thus without comment.) I was jumping back and forth between the orange keyframes, and it doesn't work when the cursor is on either of them.
- When enabling onion skinning for the object, I didn't see interpolation. In 2.81 a there is a filter, by default regular keyframes only. I changed it to "All Types" and then I could see interpolations.
This is a great tutorial. There are lots of things which need to be learned.