I think the GP meant "If the soil has been frozen for hundreds of thousands of years, how does it contain mammoths that lived < 10000 years ago".

Either the permafrost is younger than "hundreds of thousands of years" or the mammoths are older than 4000 years.

Mammoths lived from 5 million to 4000 years ago, so it could be either.

Edit: after some research it looks like both are right. Some permafrost in Yukon formed 700 000 years ago, some permafrost formed during the last ice age 10 000 years ago, some formed in the little ice age 400-150 years ago. That's not a complete list but you get the point.