Naw, you're right that the most recent 5 gig would be the easiest to recover. But what the experts are also saying is that there may still be vestiges of the previous data which could also be recovered.
Think of it this way: I write a "1" to a memory location on a Flash Drive. Then I erase it with a "0". But ... erasing it has left a "0.1" in that memory location. An expert could actually tell that it's now a zero, and that it used to be a one. And maybe I overwrite it again and it becomes 1.01.
Clearly, the normal operation of the SSD will recover the current value of the memory. But there could be fluctuations that could be read to recover previous data, even after it has been erased.
Now, that's just an example to draw out how it can occur; it isn't that straightforward in reality but you get the picture.
There may be ways to decrease the chance someone could recover data from the device, but you have to know whether you're dealing directly with memory sectors or with the memory controller (which moves data around at will), and how the underlying device behaves.
