Fascinating!

YorkiePud answered the key question.

It is indeed all “relative”.... to the Theory of Relativity, that is.

When we all watched the play live, the camera was moving forward along with Wilson, so it definitely appeared to be a lateral, and Wilson did indeed toss it behind him.

However, subsequent replays— from a non moving camera— showed that “relative” to the hash marks on the field (which are fixed in place of course) the ball did indeed drift forward.

So: Rugby rules define a lateral relative to the moving players, while American football rules define a lateral relative to fixed markers on the field.

Now, one could argue that the rotation of the earth causes the field itself to move, but that is another matter...