Of course! It's not magic - the camera pulls up the exposure in those parts of the frame to remove the vignetting which increases noise. The RF 15-35 2.8 has 4.6EV uncorrected and 2V corrected. So if you shoot 3200ISO, the corners are now like they were shot at 3200->6400->12800->19200 ISO and you still have 2EVs of vignetting to deal with.Is it really that important if the camera fixes it before you even look at the file?
And remember vignetting doesn't just affect corners. at 15mm on the RF lens, you lose a stop by the time you get to the APC corners. Here's a comparison to the Sigma at f2:
Review vignetting (peripheral shading) test results with f-stop contours for the Canon RF 15-35mm F2.8 L IS USM Lens. Compare the results from this lens with other lenses.