Why is this not Wing Chun?
Suppose for a moment, in the minds of the people in the clip, they really were doing Wing Chun.
It’s irrelevant.
Yes, that’s right: What they are thinking is irrelevant. However, what would be relevant is if the people in the clip used the same methodology of Wing Chun to learn the attributes expressed in the clip. But, a few people who are successful using “banzai!!!” punching and getting their hands up does not validate anything in different training methodology.
Birds fly by flapping their wings. Donning feathers and flapping your arms wildly will not get you off the ground regardless of the fact that’s how birds do it.
People with no training in Wing Chun manage to provide proof the effectiveness of a Wing Chun methodology? Not likely, IMO. And, I believe this holds true even if the rumors of the trainer in the clip is, or was at one point, a practitioner of a particular Wing Chun variant.
Systems are defined by the methodology used to develop the attributes necessary to express the skill. Just because something from the clip appears to have a familiar signature does not make it the same or even similar. Notwithstanding the clip giving us a chance encounter of such a signature (and any clues it may or may not give you as to whether you are on the right path), if you are using an entirely different methodology to get there, it is not a valid comparison. Such comparisons are, in fact, irrelevant.
To get back to the picture I posted in the other thread, there is hardly an accomplished practitioner of the Wing Chun style who would look it not say: “Ooh, TanDar and LinSiu DaiDar”. But the point is Felix Trinidad does not train Wing Chun; he trains boxing. He is not practicing SiuNimTao, ChumKiu, BilGee. He is not hitting a wooden dummy. He is not pairing up and playing ChiSao, ChernKiuSao, ChiGerk, or any other Wing Chun drill. So, unless you train like Felix Trinidad, there is no evidence to suggest the results of his methods (like what you see in the picture I’ve relinked here) are proof that your methods will produce the same results.

FWIW: I went to the same portion of the clip Victor is speaking of and ran it through every video filter I have to enhance the picture. There is just not enough resolution in my copy of the recording for me to make a 100% determination as to the fist orientation. However, I agree, given the upper body positioning, it is more likely than not vertical.