I just noticed the += shorthand doesn't work the way I expected with comma separated values.

For example:

x,y,z=1,2,3
x,y,z+=10,20,30

Sets x to 11 as expected, but y and z are set to 20 and 30 respectively, rather than 22 and 33.

Should the last line be equivalent to:

x+=10
y+=20
z+=30

?

Or is there some reason I'm missing?

it is not a generic (vars)(operator)(vars|values) statement.
so yeah, works only for assignments

On top of that, the += operator is not standard Lua, so it's just what zep implemented himself.