I mean if this story is to be believed, AWS reduced the bill from 6500 to 1800.
I think developers accidentally racking up unexpected thousands in costs on their first AWS project is a pretty common phenomenon that their support has standard rules for handling.
I do think the discount is believable, but we don't know the line items AWS applied a discount/removed charges.
The developer said the agent deployed multiple CloudFormation templates, I'd bet that AWS waived the charges for the unused resources - like EC2 instances that were idle most of the time, very high margin SKUs, etc.
Now, for 100 Gbps of egress (which didn't actually happen) - and this is grounded speculation - I don't think that AWS would give a discount that is greater than CloudFront rates.
100 Gbps is A LOT of data.