FDCPA 809(b) does not list an itemization of the debt as a requirement in a debt validation request.
As with any interpretation of a broad or vague statute, it is left to the courts to interpret the intent on a case-by-case basis.
Some jurisdictions have case law supporting the interpretation that a DV request can impose a requirment to provide an itemization of the asserted debt, while other jurisdictions do not.
The DV process is not, and was never intended to be, a full legal fact-finding process requiring full supporting documentation/evidence.
The same applies to the dispute process under the FCRA.
Sections 809 and 611 require an investigation and a verification based on that investigation. They do not specifically require that the debt collector "prove" the amount of the debt in order to verify the debt or accuracy of their reporting to a CRA.
A consumer always has the ability, if they contest the debt, to bring their own civil action, which then provides the ability to use the pre-trial discovery process to compel the production of all relevant evidence, such as an itemization of the asserted debt.
That will likely require consideration by the court of the contract that created the debt in order to determine whether any asserted amount complies with the provisions of FDCPA 808(1).
