This is why I said that I understood that it was just a small, slow-motion clip. None of these guys are locked up with their man. Three of them have the ball carrier dead to rights, a fourth is too wide to get in on the fun, Williams is jogging across the screen away from the ball carrier and #33 is standing there watching it all happen. From the look of the clip William's head is turned toward the ball carrier but he's moving away from him toward the sideline. Had the player who hit him went higher he'd have still been obliterated. He didn't even seem to be aware that he was running right into an opposing player.
Three other Vikings are making the play and Williams is no more involved in it than #33 at the bottom of the screen. They're both slowing down and watching their teammates make the tackle (from the look of this small, slow-motion clip)
It is something of a cheap shot hit. I'm not saying it isn't that. The 49er isn't a little guy. He could have just wiped out Williams or avoided contact entirely (might have in the regular season but who knows, preseason, jobs on the line maybe?) but he didn't.
But I've heard for years almost every season when someone gets blown up like this that you have to protect yourself out there and be aware that until the whistle is heard you can get hit, blocked, or hurt. It's just a bad idea to be jogging into the path of another player without looking where you're going during an NFL play. Nothing good comes from that.