Account hacked, still charged with bill

I created an AWS account 3 years ago for a college class then never used it since. In December I got an email saying there was potential fraud usage on my account and I saw that I had a $7,400 bill charged. I followed the instructions on the email and changed my password and submitted a ticket to report the fraud usage. After going through and deleting the instances that were created they sent my case to the billing team. After a few days they reviewed it and said that they can do an adjustment of just under $6,000 as a once off, meaning that I still have over $1,000 that I would have to pay. I asked if there was any way I could dispute the charges and they said no and closed the ticket. I've reopened another ticket to further dispute these.

There are numbers of other forum posts saying that users had entire fraud charges cleared, why would mine only be part of the charge? What determines the adjustment that they came up with? You can see in my account that nothing was used for the past 3 years and a random spike in thousands of dollars while my billing information was out of date.