Yeah you have to milk them, they get infections in their udders and other issues if they aren't milked regularly. He might've got some unexpectedly good cows. This happens sometimes. It's better to have more of your quota, than less. The feds will take your quota, or reduce it. So you have to overestimate. IDK if 30,000 litres is excessive but it doesnt sound ridiculous for a big operation. My assumption is that was 30,000L over the season, so every time he delivered he's dumped some.
IDK anything about dairy though that's just what I gather from hearing the old timer chat. This was the same way tobacco quotas worked, which I know slightly more about.