The Acoustic Symmetry thing is stated to match each earpiece at +/-1dB. But this doesn't really mean that the W4s can't have this. In fact, they had probably meant that they did check for the W4R more carefully, and that the W4s had more variance. Not to say that the W4s can't be matched perfectly, just that they aren't as consistent. If you still have both on hand right now, I would really like to see some blind ABX tests/volume level measurements to reaffirm this. You would probably be one of the first to do so!
As an aside, I just attempted to measure the volume on each individual earpiece with some cardboard, foam, and a SPL meter. While not entirely accurate by any means due to the nature of the gear, they did manage to stay within +/-1dB of each other from what I saw. Maybe I got a good pair?
Well anyone could tell the difference if they listen to them side by side, for some it could be a majour issue of not having both the drivers on the W4 balanced perfectly, but unfortunately it exists, it will be more prominent to you as how much you use it, and how long you use them for, until you notice a difference, I have tried my other friend's W4, and I still can say that one driver is indeed stronger than the other, That is why the W4R has been introduced to overcome this issue, I can confirm that it is nuetral as it can be on the W4R, and is balanced perfectly, the exact Acoustic Symmetry Westone is talking about, Hence the guy from the video states that Westone checks each driver of the W4R so they match the drivers perfectly.