Try to think of the column alias as a very simple expression

So Col1 AS X = 1 * Col1 AS X

In queries you can sort by expressions AND by the original table columns, even if they are not included in the output. If I include the original column in the output, it should become more clear:

SELECT a AS x -- "Computed" column

,a -- "Original" column

FROM(SELECT 1 AS a) d

ORDER BY

x

,a

[Edit: Added query comments]