iMovie "Join Clips" option greyed out

Articles, Videos and similar posts / questions all describe being able to join clips in iMovie using the Join Clips option in the Modify menu of iMovie (10.1.8) on macOS High Sierra.

Yet, iMovie is unable to join two clips!

It seems to be a common and recurring issue to which I am seeking an answer once and for all.

Background
I have split a clip into 3 pieces and deleted the middle part. Now, having used CMD to select the two remaining pieces - highlighting them in yellow borders, I go up to the Modify menu to find that the Join Clips option is unavailable (greyed out).

Question
Why am I not able to join the clips? They are adjacent to one another, have no speed alteration (e.g. slow motion) and are both highlighted. How do I join them?

Note: Join clips option in iMovie cannot be used for joining two different imported video clips. It can only be used to join clips created by splitting an imported clip in iMovie.

So, essentially if you have two clips which are to be combined, click on the Share icon in the Top right corner, then click on the option File and export all the clips as a single video file. You may then re-import it into the project if required after the share is successful.

For a quick 2 min demo, watch this: https://www.youtube.com/watch?v=8Px_I5fvJbk