If I am not mistaken, there is a little mistake in the video.
When Rick does the animation for the gravestone he animates the scale of the entire gravestone and not just the body part.
That means that the box collider 2D (preventing any other defender to be put on the same square) is also animated and will at some point of the animation be small enough that we can add another defender on the same square.
I hope this might help some people to avoid this little bug ^^