> As someone without any diagnostic training, I would also think that someone that is in a depression (A) could write a blogpost like that (B). That is, I can imagine that A ==> B is true.
What you have stated in words is rather more like, in the modal+temporal logic sense, "it is possible that A and then B" (temporal 'then'), not "if A, then B" (logical 'then'), which is how I read A ==> B--unless you meant, as I think you didn't, everyone experiencing A will do B.
So even in the optimistic case where, lack of diagnostic training notwithstanding, A ==> B, even then observing B does not necessarily imply A.
Which probably is part of the reason you shouldn't diagnose people over the Internet based on their posts, but via a diagnostic process actively involving them.