TIC-80 is based on real LUA on the contrary of PICO-8. And LUA doesn't support shorthands.
If you want to use shorthands like +=, -=, *=, /=, %= you should try moonscript which is supported by TIC-80
Yeah, I get that. I'm pretty sure pico uses "real" lua too, It just has some of the standard library inaccessible on purpose, to keep it in line with their philosophy of an old ass computer. I'm also pretty sure it accomplishes its three, incredibly useful, short hands by parsing your code and modifying it before passing it to what ever handles the lua calls in the engine.
Those short hands, the "IF (NOT B) I=1 J=2" if statement, the unary math operators, and the "!= " so much more readable than ~= in the low-fi font, would be nice to see in tic. But i love tic, I'm just griping.
Also, something about moonscript just turns me off. I don't like its syntax at all, it looks like an interactive fiction language like Alan or Inform7.