First, their intelligence is on part with that of Pan troglodytes (the common chimpanzee), and second, The extinction event was triggered by SCP-1000 contracting an anomalous "pseudo-disease" classified as SCP-1000-f1. This disease is passed on at the genetic level and affects every present-day instance of SCP-1000. The majority of SCP-1000 instances are born immune to the effect; those who are not born immune quickly die. The effect of SCP-1000-f1 is as follows: Any hominid (including humans, chimpanzees, bonobos, and non-immune instances of SCP-1000) that directly or indirectly observes any instance of SCP-1000 has a minimum 2% chance of being instantly killed through anomalous means via permanent cessation of brain function. This percentage is cumulative, and the longer a human views SCP-1000, the higher the chance of instantaneous death increases, at a rate of +1% chance per 20 minutes of viewing. This effect varies between individual members of SCP-1000's species, with some individuals carrying a 'death chance' of 90%. The effect is also produced by dead individuals, though small fur samples do not exhibit the effect.
Lies. Now, for the truth; there intelligence is NOT on par with that of the common chimpanzee, but with humans. and while we were still wandering hunter-gatherers, they… changed. Like we would, a few thousand years later. Tools. Weapons. Agriculture. Domesticated animals. Stable settlements. As humanity blinked in the Pleistocene sun, SCP-1000's population exploded across the night. They blanketed the planet in the tens of billions. They made things that we still can't comprehend, even though we've thoroughly studied the surviving pieces. Organic technology. They made trees and birds of prey grow into fast-moving ships, herds of animals that became trains, bushes that became flying vehicles. From insects and pigeons they made things equivalent to cell phones, televisions, computers. Atomic bombs. humans were rare, like gorillas now, a few hundred thousand left at best. We avoided their settlements just like wild animals today avoid ours. SCP-1000 understood we were intelligent like them, but avoided us just as we avoided them, saw us as fairies, as gnomes, ascribed us supernatural powers, said we ate bad children while they slept in daylight. They fenced off our dwindling wild populations in conservatories, outlawed poaching but in the underground consumed our bones as aphrodisiacs. But then their time...came to an end, and humanity was the cause. but the story is muddled. Supposedly a trickster forest god showed humanity favor, showed us the master's tools and how to use them. Why we did it, we don't know. Perhaps they hunted us, perhaps we were simply afraid. Perhaps it was just that they fenced us in, unintentionally or not. We simply don't know what the truth is. Somehow we acquired SCP-1000's own technology, and with it, we instigated an SK-class dominance shift in which humanity became the dominant species of Earth. We wiped out 70% of SCP-1000's population in a single day. Then we hunted the rest down. But we went further than just killing them. With a few of the more twisted of SCP-1000's devices, we drove the survivors mad, even those hiding beyond our reach. We trapped them in their own minds, blocking higher functions and leaving their bodies to fend for themselves like any ordinary ape. We slaughtered their living machines and burned their vast shining cities with SCP-1000's bioweapons that reduced everything to slurry and dust that washed or blew away in spring rain and wind. We left no traces. Not even our own memory. We turned one of the weapons on ourselves, wiped out any knowledge of SCP-1000 and the greatest civilization the planet had ever seen. Only a few humans protected themselves from the effect, kept the forbidden knowledge, just in case. The rest of us went back to being hunter-gatherers, none the wiser. currently, SCP-1000 are somehow regaining their forgotten intelligence and knowledge.Maybe they never truly lost it. We don't know.This is why the ever-increasing number of "Bigfoot sightings" is so worrying. Why the attempts at contact, however indecipherable, are even more worrying. Yes. SCP-1000 are just like us. That's what makes them so dangerous. We wiped them from history and memory. We dissolved their civilization and we slaughtered most of their species. Just ask yourselves: If they got the chance, what more would they do to us?
A rough translation of a recent attempt at communication by an SCP-1000:
"We forgive you
Given choice for now, not forever
Let us in"