Clearly, it's polite to respect someone's wishes when talking to or about them, whether it's pronouns, nicknames, personal space, or umpteen other things. If you're introducing Chelsea to another friend, it seems only civil and natural to say "she [not he] was stationed at Fort Drum!"
However, I don't think a person can unilaterally require that the world around them retroactively change all references to a historical event, particularly when it renders some of those details nonsensical or wrong. History is written: documents were released by a person calling himself "Bradley Manning" and identifying as (and meeting biological criteria of) a male. Other aspects of the story -- from Manning's all-male combat team to his quarters in the USDB -- hinge on that fact. Manning's presence doesn't imply that the military unit was mixed-gender. One version reports the story as it was known to everyone involved; the other modifies it after the fact according to the unilateral wishes of the subject being reported on. That's not a good pattern, I don't think.
As an outsize example, if George W. Bush transitions to a female this year, the US will not celebrate that we've had our first female president.
I'm completely sympathetic to interpersonal civility, but empathy goes both ways; I think it's beyond the pale to expect society writ large to scramble, under penalty of being called bigots, to conform to what's frankly an unverifiable declaration of internal personal change on the order of a religious awakening (which oddly doesn't enjoy the same umbrella of unquestioned sanctimony).
> At the time of the leaks Chelsea Manning identified as Bradley, a male.
Manning's first discussion with a gender counselor about identifying as a woman was a couple months before her first contact with WikiLeaks; presumably there was some time lapse between recognizing the identity and talking to a counselor. So, from the evidence available, I don't think it's correct that Manning identified as male at the time of the leaks.
Not all native English speakers got an American English education.
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.
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.
[1] https://www.merriam-webster.com/words-at-play/singular-nonbi...
https://en.wikipedia.org/wiki/Singular_they
Whether it's applicable in this case is another question.
https://en.wikipedia.org/wiki/Gender_neutrality_in_English#P...