Net-importer does not mean they import minerals. Quite the contrary, they export all the excess stuff they do not need and only net-import materials they cannot produce themselves. Most minerals do not belong into that category. T2 materials, Mexallon, ice products like Heavy Watter mostly drive their net-importer values. I see dozens of freighters of theirs going from Khanid and Kor-Azor towards Jita with hundreds of millions of units of Pyerite, Tritanium and Strontium, Liquid Ozone and other things at least every 2 days.
And in case you doubt me: How do you Dissrupt Delve