To solve more and more katas. And then, If you came back here, you'll think this kata is super easy ;-)
Of course, google is your friend ;-)

You are right, only array and object (in es6 Map, Set) can return multiple values.
Third line only return stack and is same as:
  stack.push(oper[el] ? oper[el](+stack.pop(), +stack.pop()) : +el);
  return stack;

The comma operator allows you to put multiple expressions in a place where one expression is expected.
You should look for "comma operator javascript" in Internet.