Note that the uncorrected image shown by Mr Frost in the video doesn't just show vignetting. It is also noticably wider, a good deal wider than 24mm it seems. I'm not using any filters beyond an ND filter and a CPL. Still, I don't think widening the field of view is a common defect when using filters on wide angle lenses. To support your suspicion that this review is fake because it shows something you don't expect from Canon, could you please show us an example of a filter causing such an effect?
I'm not saying this is a problem or that Canon is doomed for using such means to achieve their design requirements. But it seems like a new design mentality, from Canon at least. And dismissing this review because it shows an effect that somebody else didn't reproduce seems a bit quick, when we don't know the details of how each result was achieved. I posted my suspicion
in the other thread.
Explanation of the attachment: I took a screenshot from the corrected and uncorrected frame from the video and put the upper left corner of both versions side by side in Photoshop. I flipped the uncorrected image, so that the part of interest is right next to the other one.