Wouldn't that be true of infix syntax as well? For example, if I define an operator, %, which divides one number by a second number and then multiplies it by a third, you have to know its arity to parse:

  70 % 90 100 * 10

Even worse, in this case you're also dealing with operator precedence, which in (Reverse) Polish Notation isn't an issue.

Yes, but infix syntax is usually used only for a few select functions, whereas Forth uses postfix syntax for all functions.

In Forth parsing doesn't even come into play, because an operator just tries to pop as many arguments off the stack as it needs. Annoyingly, that does mean there's no way to tell what the following:

  a b +

Means, as it can be any of these, for example:

  add(b, a)
  add(b(a))
  add(b(a()))
  add(b(), a())
  add(b, a())
  add(b(), a)

In a language where you don't need brackets for function calls it can be reduced to:

  add b, a
  add b a

And even if the add function takes two arguments the second option would be fine because the call:

  b a

May put two arguments on the stack. add(b(), a())