Though I have no particular knowledge of this event, ferry fuel systems in general can be difficult, and this is not the first time an aircraft has taken unuseable ferry fuel into a ditching. When I have ferried with extra tanks, I have been aware of the critcality of vents working as intended, because for a gravity feed tank, if the vent air cannot get it, the fuel won't get out. Lots of attention is paid to the supply line, but vent lines are an afterthought.
My first time ever flying a Twin Otter was an hour on my own in right seat shortly after takeoff, while my captain was in the back disassembling the venting system to clear a vent line, so we could use that fuel. He did succeed, others have not. Had he not, we would have had an overgross landing, though likely on a runway, as opposed to the ocean.
For that reason, I'm a real fan of TurtlePac bladder tanks, as venting is not required. The problem is that old guidance material still seems to favour rigid tanks, so that's what are often used, and venting becomes a critical consideration.