It's an interesting story. Here's a couple of my thoughts.
An interesting finding is that: "The researchers also showed the long-term memory never matured if the connection between the hippocampus and the cortex was blocked." And the fear stimulus is a simple thing to remember. So a more accurate headline might be at least some long term memories are created in parallel. The long term memories mature over time and and this process required inputs from the hippocampus.
"Our model (Fig. 4L) introduces the concept that the prefrontal memory engram is already generated, albeit in an immature form, on day 1 of training..." Can an immature long term memory engram be 'rewired' during the maturation process, so that training makes it into something that was never really experienced?