After a bit of surfing, I found this DANFS item on USS BERNADOU (DD-153), the destroyer that took aboard the gunners and American army officers traveling on MS JAGERSFONTEIN before her sinking.
It reads in part, "She [BERNADOU] conducted antisubmarine warfare exercises on 26 June 1942, but news of U-107's sinking of SS JAEGERSFONTEIN cut them short. On 27 June, BERNADOU arrived on the scene and took 86 United States Army officers and 13 British and Dutch gunners from SS ST. CERGUE, which had reached the scene of the tragedy first. After taking the survivors on board, she returned to Hamilton, Bermuda, arriving the next day."
My conclusion from the Dutch and British gunners assigned to her, rather than a USN armed guard, is that, unlike the three other modern Dutch merchant ships you named, JAGERSFONTEIN was never under the control of the U.S. War Shipping Administration.