This can be true after he has played at least half season as a CB without many errors.
Right now we know that he won't ever make as a CDM, that's why Pep used him as a CB, where we lacked players due to holidays. Stones, Ake, Akanji, Ruben weren't available.
As a CB Kalvin didn't have to receive the ball under the constant pressure, compared to CDM position and had more seconds to make decision and pass the ball to someone. That was the difference and that's why he looked there better than he looked playing CDM.
But CB is measured by stability, 10-15-20 games and therefore Kalvin has to complete that path, before any verdict can be made about his abilities as a CB.
Even Fernandinho struggled there in 19/20, when Pep switched his role. So I won't be so sure about Kalvin as well.