For the RF 14-35mm, the uncorrected FoV is slightly less than 13mm. The distortion correction reduces the FoV, but because the image is 'stretched out' (after being 'squished' by the distortion), not through cropping. Having said that, there is some cropping at the sides to keep the 3:2 aspect ratio, because there's more stretching on the longer axis of the image, resulting in an EOS R output that's 305 pixels (4.5%) wider than the native 6720 pixels, and those extra pixels cropped away by Canon/Adobe (but optionally not by DxO) to keep the 3:2 ratio and native output dimensions.
All of those details (and more) are included in my tests of the 14-35 at the wide end:
Mirrorless cameras lack an optical viewfinder, so looking through the lens is always an electronic process. This fact enables lens manufacturers to include consideration of digital optical corrections in their lens designs and make trade-offs accordingly – more optical aberrations can be...
www.canonrumors.com
Simple explanation for the cropping in the article you link. "
Supreme Court of Ohio – Distortion Corrected and perspective corrected," then, "
The uncorrected image is also a fair bit wider than 14mm, with the field of view reduced a little when correcting the distortion, however, natively it’s not quite as dramatic as shown in the second image, as this also was perspective corrected for keystoning, further reducing the angle of view."
Unlike the site you link, in my tests I ensured that the camera was level and orthogonal to the building to avoid vertical and horizontal distortions. Perspective correction can result in substantial cropping, depending on the amount applied. Here's a rather extreme example of that, taken with the RF 14-35/4 @ 14mm as I walked past the Parthenon in Rome.
Perspective corrected shot:
View attachment 212206
Original shot:
View attachment 212203
Perspective correction step (force-rectangle in DxO), showing how much cropping is needed after the keystoning is corrected:
View attachment 212205
One more point regarding focal lengths. Even though the 14-35/4 gives an uncorrected FoV equivalent to ~13mm, the actual focal length is still 14mm (slightly longer, since Canon always rounds in their favor). Consider that the diagonal AoV of the RF 15-35/2.8 at the 15mm is 110°, while the diagonal AoV of the EF 15/2.8 fisheye is 180°. Both are 15mm, but the first is neatly rectilinear (distortion mostly corrected optically), while the fisheye isn’t corrected (circular projection, massive ‘black corners’). Think of the RF lenses requiring distortion correction as one step down the path from rectilinear to fisheye.