Simple fact is that if they were able to identify the City fan who did this:
https://www.manchestereveningnews.c...-news/moment-madness-landed-city-fan-13694742
then they can just as easily identify the Liverpool fan who chucked that pint pot full of coins. As you say, they know pretty much exactly where it was lobbed from in the away section. If there's CCTV footage of the offender throwing it, they'll have the row and seat number. Liverpool FC will be able to advise who that ticket was sold to. Granted, there may be caveats here - fans don't necessarily always sit/stand in the seat they're allocated (often I'll move around a stand at both home and away games and stand with my mates), or the ticket might have been passed on to someone else to use (in that case, the original purchaser needs to be reprimanded if they can't find the offender).