Any transfers resulting from the application of Section 5302 are effective by reason of the account contracts involved and this part and are not to be considered as testamentary. The right under this part of a surviving party to a joint account, or of a beneficiary, or of a P.O.D. payee, to the sums on deposit on the death of a party to a multiple-party account shall not be denied, abridged, or affected because such right has not been created by a writing executed in accordance with the laws of this state prescribing the requirements to effect a valid testamentary disposition of property.