The guy should've put a quarter on the glass but it's a dousche move to stack a bunch of credits on a machine in a public place. Now I understand there was nobody there but it doesn't matter IMHO. Eventually somebody will walk in and You were still hogging a machine in a public place. I would've told the guy to jump in and just give me cash to buy half my credits and play for some beers and have some laughs. It's sounds like he really wanted to play the same machine you were playing. This kind of thing has been going on for decades in pinball so in the big scheme of things who cares. No big deal.
Some guys also just like to play by themselves and zone out too. It doesn't sound like he was a very social person either.