I think it's even simpler. Starting a subquey here doesn't make sense since there is no main query. So the parens gets dropped promoting this query to main. Same with the union

ORDER BY actually happens last.

    jamie=# (select a, a+1 as c from nums) order by b,c;
     a | c 
    ---+---
     3 | 4
     0 | 1
     1 | 2
     2 | 3

(4 rows)

It's a separate part of the grammar:

    <query primary> ::=
        <simple table>
      | <left paren> <query expression body> [ <order by clause> ] [ <result offset clause> ] [ <fetch first clause> ] <right paren>

But, if "QE is a <query expression body> that is a <query term> that is a <query primary> that is a <simpletable> that is a <query specification>" then the <order by clause> may select columns from the <table expression> even though those don't really exist any more by the time the <order by clause> runs. It's tricky to implement.