The conversation in some aspect may have tripped the flamewar detection, which removes the reply button iirc. You can manually reply to a specific comment by clicking/tapping on on its timestamp.
In the context of the union query, I think it makes sense. The query takes two different columns (with two different names) from the same table, and combines them together into one column. The TS error happened because the column names didn't match between the two union inputs. But they would never match without the "as".