Is leaving the site for the duration of a warn a sufficient way to have warn removed

No, it does not.

How come it always works for then? How can we get his 30% reinstated that he has shed after several months of afk requests

I've witnessed multiple people completely stop posting for months, sometimes lurking others just not log in at all, and when they come back they get warn removed. Even once say someone who basically quit jsp and crashed out hard before leaving, only to come back and instantly get 20% removed at once.

As an ex warn offender i can confirm that simply afking was most efficient way of removing warn.

It is very odd that the facts do not match these statements made by administration.