My enemy is just outside to the right portion of the screen, and it didn't move at all, just like when it moved to the left on its own. So I didn't found necessary to include such line in the _ready function.
After adding the visibility enabler node, no, it is not necessary anymore to set physics process to false in the script. This is because the node will take of toggling it for you. But without a visibility enabler, you can use the ready function.
It might be useful to re-record this video since
Sure, there's room for small improvements. Note however this course isn't going to get its videos remade at this point; we're working on a new programming course for beginners instead.
But I'll add notes to the video, thanks for your feedback.