Not sure about that third point. Ruby desugars += and similar compound operators into separate operate and assign steps. There's no need for an implementation of += in addition to +. I'm sure Lua could do the same thing.
My guess for why Lua doesn't provide a table copy function is because then they'd have to decide whether to provide shallow or deep copies, and deep copies have to do cycle detection and other such stuff. Then there's metatables, functions, and so on.
I think Lua accounted for the idea of desugaring, where PUC-Rio said that they didn't want the added complexity, but I don't think I could readily find that justification.
This is just another one of those situations where Lua is not catering to anyone by randomly catering to one group or another. No one is happy.
My guess for why Lua doesn't provide a table copy function is because then they'd have to decide whether to provide shallow or deep copies, and deep copies have to do cycle detection and other such stuff. Then there's metatables, functions, and so on.