because left || right actually compiles to something like

case left do
  v when v == false or v == nil -> right
  _ -> left
end

Which is not allowed in guards

Only || and && are not allowed in guards. All other boolean operators are allowed

Yeah && and || have what is called a “short circuit” behavior where they will only evaluate the right hand side of the operand if they need to. This allows you to do things like:

value = params[:some_opt_key] || raise "this parameter is required!

If the left hand side is “truthy” then || short circuits and doesn’t evaluate the raise at all. This sort of branching behavior doesn’t even make sense in guards.