In the original timeline before that episode the Federation was more pacifistic and hasn't been in a war for a long time, the Klingons were Federation members and there were other smaller things that don't make sense with what we learnt after that episode.
My theory is that Tasha Yar joining the crew of the Enterprise-C and travelling back to the battle of Narendra, when she originally wasn't there, changed a lot of things while others stayed the same.
A hint for my theory being true is that most canon discrepancies came up after that episode, while the show before "Yesterday's Enterprise" was mostly consistent with itself with its background information.
My theory is that Tasha Yar joining the crew of the Enterprise-C and travelling back to the battle of Narendra, when she originally wasn't there, changed a lot of things while others stayed the same.
A hint for my theory being true is that most canon discrepancies came up after that episode, while the show before "Yesterday's Enterprise" was mostly consistent with itself with its background information.