It is same logic as

SELECT * from #t

WHERE NOT ISNULL(id,0) = 1

or

SELECT * from #t

WHERE ISNULL(id,0) != 1

Both the queries will produce same result, since in first query = has greater proirity than not it will check "ISNULL(id,0) = 1" and apply not to that ..

but later one is clear in understanding.