Do I have to admit I owe the money when a debt buyer sues me?

No, and denying based on lack of sufficient information, a real, formal option under California procedure, isn't dodging the question. It's an honest answer when you genuinely can't verify what's being claimed.

That's an ordinary, expected position to be in once an account has passed through one or more debt buyers after leaving the original lender. You have no direct way to confirm numbers coming from a stranger to the original transaction, and the law doesn't ask you to simply take their word for it.