Yeah, I recall that's why ZeRo didn't call it cancelling. Regardless it lets moves like fsmash come out faster by doing a turn around animation first. By walk in the opposite direction I mean face one way then pull the stick in the opposite direction. It makes him walk faster.
I am pretty sure you are mistaken in both accounts.
Turning around doesn't cancel the lag, it cancels the animation; the lag is already over. What I mean is, F-Tilt IASA frames are before Roy's animation returns to idle so you can input anything, you do not need to turn around to do so. There is no technique or anything involved, it's just the move has more frames animation Roy moving than it has frames that do not allow you to input a move.
As for the walk thing I think it's just your mind playing tricks on you because Roy has a very janky pivot in place animation which makes him slide without moving his feet so he looks like he is gliding the first frames of the walk after a pivot. He seems to cover distance at the exact same speed though (animations aside).
Try not to let animations in the game fool you about the mechanics that lie underneath. Sometimes animation aren't true to what's going on. Certain chars (like Mewtwo for example) have moves that do not have hitboxes where the moves show they do, or certain other have hitboxes where they don't show they do. Certain moves have lag even after the character is already in idle, and certain others (like Roy's F-Tilt) have the lag end before the animation does.
I think the easiest way to test this is making a huge stage and just timing yourself in 1/4th speed or racing 2 Roys concurrently. I just did both of those tests (did each test twice) on the biggest stage that I could make (in stage builder) and on both occasions the one that wasn't facing the opposite way was very slightly faster but it was a difference of 0.3-0.4 seconds in 1/4th time which is due to the fact that he didn't have to pivot before starting to walk. You are welcome to try yourself and prove me wrong though, I could have made a mistake, we all do.