According to the linked abstract, the study considered how to change fridge doors. In other words, the question being answered was, "What style of door is easiest for children to open from the inside?"

Given that many (most? all?) fridges of 1958 had locking latches that opened only from the outside, it's not inconceivable that moving to a magnetically sealed door like modern fridges was not seen as a trivial, or the most obvious, change at the time.