question:
i keep hearing references in horus books about tyrannid teeth, bones, or trophies. i hear about machine spirits and et al in these 30k, age of reason books.
what gives? it seems almost sloppy for them.
Machine spirits were a thing even in 30k. There were skeptics in the Mechanicus, but only a handful, the vast majority of the Cult believed in the existence of spirits in even the simplest machinery, and basically everyone else took them at their word, even planets the Imperium hadn't found yet believed in machine spirits because they had been in contact with Mechanicus Explorator fleets and settlements during the Age of Strife.
Trophies were just a common thing among the warrior cultures most of the soldiery came from. Most of the original Astartes were recruited from the nations that were formerly at war with the Emperor, many of which were still ruled by warlords who had capitulated to him and become part of the Terran bureaucracy. Marines kept them as mementos of foes that seemed significant when they beat them, human soldiers as something generally more primitive, like taking scalps or bones to steal some of the dead foe's power. The Emperor vaguely encouraged it, knowing of the totemic powers such things can have, the Primarchs likewise approved, and then the warrior lodges that became common among the military encouraged it further. Most marines had a small collection of trophies claimed from xenos and rogue human elements by the time of the Heresy.
Not sure if you mean you genuinely saw a reference to Tyranid teeth in HH, if you did that was either an anachronism or a reference to Hive Fleet Colossus, though they weren't known as Tyranids because Tyran hadn't been eaten by them, so the name would be anachronistic regardless.
It's important to remember that even the time of the Emperor was one of superstition and myth. Terra had only barely been lifted from techno-barbarism and wars between rival clans and nations. Most of the population was still nomads that bore a resemblance to something out of Mad Max, Mars had been soaking in indoctrinating waves from the Void Dragon for millennia, everywhere else had been ravaged by daemons, aliens, civil wars, insane AIs and stranger darker things on occasion. Knowledge and wisdom had been forsaken in the desperate scrape to survive and placate horrific beings.