In British politics, the shadow foreign secretary is a position within the opposition's shadow cabinet that deals mainly with issues surrounding the Foreign Office. If elected, the person designated as shadow foreign secretary may be slated to serve as the new Foreign Secretary.
No one in British politics has ever been a foreign secretary and then later become a shadow foreign secretary.
There's nothing to prevent a former foreign secretary from becoming a shadow foreign secretary if their party loses power.  So it could happen, but it's not known from the text alone.  So this isn't definitely correct or incorrect.  The system seems to think this is definitely incorrect, meaning they don't understand the unidirectional nature of time.  They seem to think it's assured that someone has gone from full secretary to shadow, but the article only implies that it can happen the other way: shadow to full.  I guess the computer thinks that A-to-B is the same thing as B-to-A, when in this case it's not.