They said said the blockchain is "easier", not that cash is untraceable. It would require the participation of innumerable businesses to properly track cash; as it is, they function as a huge network of crypto tumblers.
You can't track all transactions done with cash, you probably can pick a bill from 10 years ago and locate the bank it originally came from but you can't know all transactions done with that bill.
There are literally serial numbers on every bill printed in the United States.