This secrecy was taken so seriously, in fact, that Jordan himself "had to lie to [his] family" about his return.
Michael B. Jordan's Killmonger Change
Director Ryan Coogler discussed in Black Panther: Wakanda Forever's Director Commentary that Michael B. Jordan (Erik Killmonger) was responsible for a certain character detail in his cameo appearance.
During his pivotal scene, Killmonger appears with his hair tied up — a choice Coogler revealed to be courtesy of Jordan himself, "so it could feel like as if it was Ramonda's hat:"
"And then big shout-out to Mike B. Jordan, because he had this idea for his hair to be up so it could feel like as if it was Ramonda's hat."
During this scene, both Shuri and the audience are left questioning "why would she see him and not these other people," like Ramonda:
"And this whole scene's about the purpose of why would she see him and not these other people, you know."
Having what appears to be a distinguishing feature of Ramonda's be visible prior to the reveal that someone else is sitting on the throne helps put that key question into focus. Further, it is possible that it is meant to emphasize that despite Shuri not seeing who she expected to be in the chair, elements of that person are still there.
A Simple Choice Enhances the Twist
A detail this small may have been missed by many on their first viewing of Black Panther: Wakanda Forever, but for those who did notice it, it made Killmonger's reveal even more shocking.
With much of the film dealing with Ramonda and Shuri prior to this point, it would be natural that Ramonda is the person Shuri would see. Audience expectations are set, and a brilliant twist is revealed.
But, even if someone had doubts that it would be Ramonda on the throne, they likely would have been further convinced if they squinted and saw what looked like her hat.
These little details help ensure twists like Killmonger's reveal can work for even the most eagle-eyed viewers.
Black Panther: Wakanda Forever is now streaming on Disney+.