Second in the first video you posted the arm is being basically thrusted at the student in the upright position, basically striking with the forearm head on, where as, WSL makes it clear that you should be "following through" so to speak so the arm, albeit briefly, rides/slides up the opposing bong vs striking it head on. To really see both of these requires playing the videos at the slowest speed available.
It seems that rather than understand the drill through explanation from people who know the system, you're more interested in googling something to counter it for some reason.
If you search Youtube and find something you think looks different, you should ask, rather than present it as counter evidence to say something else you also don't understand is wrong.
Even at the slowest speed you are not being very observant.
In that video you also see both WSL and PB's fists point upward, but elbows drive forward, as in the OP video here. WSL then illustrates clearly how the punch is going forward, from the elbow, not hacking down on the arm like Joy's lineage.
If you are observant, you will see WSL is driving with the elbow, not leading with the fist.
I explained earlier the problem with leading with the fist and actually trying to hit them and force blocks. It is something I've experienced directly when people from other lineages have worked out with me.
They try to punch straight at me, reaching over my
bong-sau which extends their arm and lifts their elbow up, losing hip-elbow connection and resulting in their arm getting displaced way out of position by my
bong-sau. It just doesn't work as a drill anymore, and is not developing anything that way.
That's why I usually don't do
chi-sau or other drills with different lineages, except to reveal their errors to them. With different thinking, it ruins the exercise and renders it useless practice. Better to free-spar to compare.
After that, they then relearn it correctly.
First, yes the wu starts on the other side of the centerline BUT when it engages and laps it is on the centerline. Why? Because the incoming strike should be on the centerline.
This is exactly what is happening in the OP video. Maybe you are not seeing it right due to camera angle, otherwise, what is the problem then?
This goes to the point I made previously about context. We don't know the context and without that we can't even be sure what is being addressed in the first video.
You can't, because you don't know the system. If you know the system, it is very clear what is going on.
If you know the system, you can even tell in any video when errors are intentionally made to check for correct responses from the partner.
We are very familiar with these drills because we train the same system. So, better for you to not make assumptions and scour the internet for counter evidence, but instead to
listen... if you are actually interested.