Chill hours I thought were anything under 10 degrees (Celsius, of course), but I gather officially it's probably closer to 7 or 8 degrees. Sub-zero isn't necessary, in any case.
You're right that they've doubtless been selecting for quite a while at their site - and will therefore have varieties that are happy with chill hours for the region.
Though I'm surprised they get as cold as -3 given how far north, and how close to the coast (20km) they are.
Lismore - to their west, away from the coast - has an elevation of only ~10 metres. The township they're closer looks to be Wollongbar, and that's showing as ~160 metres elevation. I believe the soil around there is superb (volcanic, not depleted like 'average Australian farming land') which is probably a bigger factor.
I'm south of them by 4 degrees, same elevation, and have Much Optimism for my half dozen new blueberry plans ... once I get them out of pots. Shame that these Eurekas plants don't seem to be for sale.
EDIT: meant to add - some varieties that are popular here in AU - Sharpe Blue, Biloxi - have low chill hour requirements, around 300-400 say, BUT really don't like frosts, so it's quite the juggle.