Traditionally in US schools you were taught that applying "they" to a non-plural was outright incorrect—as in, a student would lose points if used that way on homework growing up.
When you don't know the gender, however, it is really convenient to use "they", so albeit the incorrect form, with time it started to take hold. Lately gender politics have acted as a catalyst as some people outright tell you they prefer that you use the "they/them" pronouns. Since a lot of folks are already comfortable using they/them, it works.
A different tactic is to just substitute something else entirely. Books like On Writing encourage young writers to avoid the situation and try to substitute other words rather than use "they" or just default to "he." An example might be using the word "writer". For example the books recommends instead of using "He should learn to use semicolons only when absolutely necessary" use "The writer should learn to use..." instead.
I think the latter method is harder in non-formal threads like this. "The imprisoned already has served 7 years" works, but comes across as really impersonal.
"They" is the only option when you don't know the gender of the person but it is still valid even if you do know the gender, though less commonly used in that situation
From a grammatical perspective it should be noted that "they" is plural. It is gender-neutral, but should only be used when referring to multiple people. The correct gender-neutral singular pronoun is "he or she".
I thought the same thing but there's no way a native English speaker would use "they" in that context otherwise. It made me have to go back and see if Wikileaks in general was being referenced.
It also reads as someone who understands the pronoun issue, but doesn't understand the temporal rules around pronouns. Instead, just opting for the traditional gender-neutral pronoun.
There's a lot of different perspectives and opinions, and honestly the rules aren't clear cut yet. At this point I pretty much always refer to people as "them" online.
That's a valid interpretation, but it's based on assumptions. Personally I didn't even notice the use of "they" vs "he" or "she" since it's not really relevant to the discussion. In fact, OP has clarified that he/she used "they" only because they were unsure if gender changes apply retroactively since at the time of the leaks, manning identified as a "he"
It is not other people's responsibility to know what your preferred pronoun is; you don't have a right to a preferred pronoun; you don't get to tell other people how they should speak. They is perfectly appropriate in all situations as it is gender neutral.