Understanding security better

[blockcurious]

I heard an analogy today describing the blockchain like this: If someone wants to steal diamonds, they break into Tiffany's and grab as many diamonds as they can and leave with a nice score. A blockhain would break up all the diamonds into millions of pieces and spread them across the millions of stores each with its' own security. So now the thief has to break into 100 of these stores to even get half a diamond. My question is, is this accurate? And does this apply to a public blockchain like Bitcoins or would this be how a private one secures itself? I always thought the security of it was that since the history of the chain is distributed to everyone on the network, that any bad actor who tried to change the ledger would immediately be dismissed since every other ledger wouldn't agree.