The A380 and I believe the 320 has an airspeed input into the LGCIU that prevents extension at high speed. In the 380 it’s 280kts, can’t remember the 320.
As to what they were thinking. Nothing. They had pressonitis which then overloaded their cognitive functions. The big question is why the PM didn’t intervene earlier in the error chain. With the time of year and the airlines culture, I think we know the answer to that one.