Pahanin was the first person he killed that's why he was mention, killing pahanin was the point of no return for Dredgen and that's why it's such a crucial event. Also, we wouldn't know how pahanin died if it wasn't for the cloak so that might be one of the reasons why.

If they wanted to include that Pahanin was killed by Yor, they would have put it in the Grimoire. They could have easily added it in to the one where Yor abandoned his Ghost: [spoiler][u.1:2.1] You have blood on your hands. [u.2:2.2] How’s that any different than prior? [u.1:2.2] This is not like the Crucible. Pahanin is gone forever. This is Innocent Blood [u.2:2.3] Matter of perspective.[/spoiler] But they chose to put it in the Flavor Text. If it was the point of no return (which it was) wouldn't they have put it in the Grimoire on account of it being a major part of Yor's existance? They would have, unless hit had greater meaning in the Cloak's description.

Or because they know that most of the community doesn't use the website and that they don't care about the lore as long as there's things to shoot and loot to be dropped. They're not going to give massive hints in game, just point players in the right direction.