My guess is they tried to delete it. WHs probably had a snap shot of the "before picture" and found some things when they matched it up with the "after picture" and played some spot the difference.
It makes sense they wouldn't pull the trigger on Twitter without a plan, so this was probably set up years in advance.
Also possible WHs knew about back doors because they were using them too.
Or they arrogantly believed they wouldn't find it.
Or when Twitter's code was locked down, it prevented them from doing anything.
My guess is they tried to delete it. WHs probably had a snap shot of the "before picture" and found some things when they matched it up with the "after picture" and played some spot the difference.
It makes sense they wouldn't pull the trigger on Twitter without a plan, so this was probably set up years in advance.
Also possible WHs knew about back doors because they were using them too.