Normally I would agree with you, but If you look at the datasheet of the JTK3048S05 DCDC module, you provided and look on the bottom of the last page, there a EMI input filter is shown with an inductor directly connected in series to the input of the DC/DC converter. Therefore I assume (educated guess) that there is sufficient input capacitance to handle to ripple current. But as mentioned it is only guessed.