They don't really know. This entire new theory is very controversial. We do know that cosmic material was embedded by very high velocities, in mammoth tusks and bison skulls. It seems to have happened more than once. 35,000 and 12,900 years ago. Clovis points also have the same material in them from 12,900 years ago. It is highly suspected that many animals were killed by the shockwave, the firestorm, and the high velocity material. But, there are far more mysteries than facts at this point.
We don't know if the Mammoths were alive or dead, because tusks don't grow over injuries. Bison skulls do, so they think the bison lived a while after the impact. As for the Clovis spear points, nobody knows if the Clovis were alive or dead at the time.
We do know that the Clovis people, as well as the Mammoths and a bunch of other species, all vanished soon after the suspected impact 12,900 years ago.