True, but it's unlikely that someone would, for example, see me across the street (when I have obviously not noticed them), run across and thank me for all the help and tell me how much their life has improved unless it had, in fact, improved.

I'm not convinced of this. Some form of self-deception could be in play. There's some evidence that in some aspects depressed people are more realistic and rational about their surrounding world. A person who is no longer depressed might simply have convinced themselves otherwise. I consider this to be unlikely but it is worth considering in this context.