If we assume BA’s interpretation of journey is a single journey comprised of one individual flight, their statement then makes even less sense, particularly the “whole journey” with “any upgrade”

“Any upgrade gets [benefit] for your whole journey” would be an entirely redundant statement in that case.

To me it just seems whoever wrote this has no idea how it works, and whoever reviewed it and approved it to be published is equally incompetent unless it was intended to be deliberately deceptive.