Of the few SCUMM script examples I've seen, I can't seem to understand what Gilbert means when he says: "It did things Lua could never dream of." The ability to control a C64 from a UNIX box? That actually sounds a lot like Naughty Dog's GOAL: https://en.wikipedia.org/wiki/Game_Oriented_Assembly_Lisp
I think you have to interpret that snide remark as tongue-in-cheek; he also says SCUMM beat up Lua for its lunch money. And he is playing that "grumpy old developer" role pretty consistently, which means bashing the new and venerating the old. Still, I think his main point is that SCUMM was a higher-level language than Lua that could express actions in a more game-specific form, and that's why it's more suitable (at least to his way of doing things), and that's why he would like to continue the SCUMM way rather than actually use Lua.