If there were some checkbox still checked (perhaps in some submenu that needs to be expanded), then "confirm my choices" might obviously have a different conseq

If there were some checkbox still checked (perhaps in some submenu that needs to be expanded), then "confirm my choices" might obviously have a different consequence. That might not apply here, but I am pretty sure I have seen that on other sites before. I am not a laywer, but "confirm my choices" just sounds like it could be interpreted in a way benefitting the website owner's interests in some cases even if everything is unticked, whereas "Reject all" is a very clear statement of disagreement. Similarly, you wouldn't say "I agree to everything on this list after I have removed all items", but "I disagree with anything on that list you gave me". But that's a general pattern it seems.