I agree, but we simply don't know enough. On one hand it could have been RW alone (or with Moth) who removed it during a round of edits and managed to explain it away ("oh, I got confused when typing in a rush / I meant someone else's mother / it's been such an emotional rollercoaster I sometimes forget my name, etc etc"). That's if anyone noticed or queried its removal.
However, if someone else was involved in that decision then the omission would need explaining given it was a "true" story.
Why? Because there would be no need to remove it had they believed RWs word to be true that her mother died in 2013 before the house loss, diagnosis and walk. In fact, it would provide a fuller emotional and tragic backstory with a triple-whammy of a hook: grieving her mother, becoming homeless, and facing the death of Moth. Yes, it would perhaps make the backstory a bit more complicated, but in this manuscript scenario it would be true (according to and trusting in RW) and no need to completely erase it. Keeping it wouldn't even require going into much detail about her mother's death. I mean we barely get much on how they lose the house or the diagnosis anyway.
And yet if someone else was involved in removing it then why did they do that? Why remove a "truth" from a true story that would benefit the story and understanding of RW's life (as opposed to keeping in a bit about the death of a pet sheep!), unless you knew that "truth" to be false.
And if someone knew it were false, then they must have known the real 2015 date. And so it goes they must either have been aware that Moth's diagnosis didn't happen until after the walk or they queried this alternative conflicting timeline with RW and she somehow convincingly explained it away to get it back on track to 2013.
But yeah, we simply don't know enough to draw any conclusions. All it tells us, and crucially so, is that RW originally wrote that her mother died 6 months before Moth's diagnosis, which strongly adds weight to the diagnosis being in 2015, thereby undermining TSP's central premise.