If you look at Fox doing areals in the two slides that he is, they're different. In the First slide, fox appears to be doing what was widely accepted as his Nair and it just looks different now. However, in the last slide, fox is doing an areal in the opposite direction and it looks dead on his Nair in Melee.
My theory, therefore, is that in that first slide, fox is doing his Fair. You know, the spinning kick of death that no one uses. I just can't think of it being any other move looking like that. And so far they've seemed to keep his old moveset.
*to avoid confusion, when I say "first slide" I'm referring to the first slide that fox is doing an areal*