Yes, you have that right, MalcomC. The vignette is a well-known characteristic “feature” of this lens and it is supposed to be that way. Everyone who has reviewed it has pointed this out. As Dustin Abbot put it . . .
" . . .what I saw was clearly a mechanical vignette – an obstruction that kept the corners of the frame from receiving light at all [as Perfrode correctly pointed out]. This was one of the compromises that Canon had to make to make this lens work. […] The lens actually frames wider than the physical design allows for, and then, when corrected, it shows the proper look at 24mm. […] You can see just how much latitude that Canon is giving itself for digital correction. One just has to accept that this is part of the len’s design, in that it was clearly not possible for them to go all the way to 24mm without something having to give." https://dustinabbott.net/2020/04/canon-rf-24-240mm-f4-6-3-is-review/