(glanced at it so I could be wrong) They're talking about a public key that can be used to validate the JWT's authenticity. AFAIK there is no need to keep these secret, and it's not possible to (without breaking public key crypto) forge them so it should be safe to store them wherever.

From article:

Private key redis key

    public static string PrivateKey(string kid) => $"{Root}:jwks:private:{kid}"; // full private material (short life)