Import duty was lowered from £35 to £15 very quietly. Thus constituting a stealth tax. But in a "no deal" Brexshit would help pay for the mess. Until people wised-up, and by then they would buy from UK sources and import duty would have been paid anyway. Win Win, but still underhand.
No, 'import duty' was not lowered. I think what you refer to was the lowering of the threshold for paying VAT on imports. That threshold is currently £15. The threshold for paying 'Customs Duty' (also known as 'import duty') is £135. For goods below that value no Customs duty is charged. There is also Excise duty which is charged on tobacco and alcohol at levels below that £135 threshold.
Tax and Customs for goods sent from abroad: HMRC