Teaching Lift to Seventh Graders

<<the interaction of the two counterrotating wingtip vortices with the
ground causes them to move toward each other and cancel each other out
a relatively short distance behind the plane. >>

I think you misunderstood the article.  The two wingtip vortices don't
cancel each other out.  Each vortex cancels out with the "mirror
image" below it.

Anyway, I think it's inappropriate for Denker to use this mirror image
as an explanation.  It's been used mathematically as a way to
calculate ground effect, and as a way to model it in a wind tunnel,
but that's a far cry from saying it's "real", in my opinion.