The Beato clip's taken a little out of context, Lukather says that he initially played the riff with a louder and heavier guitar tone because Eddie Van Halen was on the recording, but was asked to play it softer. The funky chords between the main riff and the verse were written before Lukather became involved, though he did change those slightly. MJ's also the only credited songwriter, I know writing credits can be a little tricky but I expect Steve would've been given the full credit if it were his.
I've seen him talk about the riff before, saying something like (totally paraphrasing) "
it's not a typical riff a guitarist would write, that's MJ singing the riff". I can't find the full quote*, but he also says
here: "Beat It was Michael’s riff, and when I changed it a little he started dancing, doing the Michael Jackson thing two feet away from me"
*While I was looking for the source, I watched the full Beato video. Lukather says at about 9:20 that Michael wrote it.