The scrubbing would; still need to be done manually... either that or they'd need to design a program to do it ...they'd have to estimate manually and go screening for people who got leaves during that time.
This program is called a "database". They've existed for about 50 years in various forms. I'm pretty sure Riot keeps their data in one. (Probably more than one.)
SELECT * FROM Games WHERE <Game.EndTime less than 6 hours ago> AND <a player in the game was flagged by LeaverBuster>
<remove all those AFKs from the database>
Or maybe something like:
SELECT * FROM Players WHERE <player got flagged by LeaverBuster today>
<remove all those AFKs from the database>
Done.
If you want to get real fancy you script it to happen automatically when Loss Prevention is turned on. Not hard.
I'm not going to argue this, because I doubt their data is quite that simple or, judging by what I've read some of their programmers say about their backend, easily worked with... and neither of us have access to it so our arguments will quickly become childish.
But the real point is that you're assuming they KNOW when the issue started, that they actually care enough to track it. If they did they'd be able to apply loss forgiven retroactively. I don't think they know exactly, and since they wouldn't want any false negatives(judging by the strictness of the system) I doubt they'd guess.
Judging by this system's stated objectives - to punish ANYONE who leaves a game regardless of cause - I simply think they'll see all those people being punished as 'working as intended' and not worry about it, honestly.