A number of under the hood elements are being reused. Telepathy still drives the messaging system, and ofono is being used for the modem. The GTK2 UI from Maemo was dead in the water even before the shift to QT when Meego was released, so it makes sense that they would base a new GTK UI on a modern Gnome. A QT UI is available in the form of SailfishOS.
Regardless, the "interesting" work on the Librem 5 sits with none of these levels, but further down the stack, with mainlining the kernel work needed for the device. After that, the userland work provided by Gnome/Plasma Mobile/Ubuntu Touch/Mer all become interchangeable.
