The point was that this was a change that required an explicit act by parliament to happen, i.e. they made a concious choice to increase the limits. The laws did not automatically revert/change when brexit was decided.
So the timeline is: UK as part of EU adopts the regulation into national law. UK leaves EU, national law is unchanged by this (so regulation persists). UK makes a new national law changing the amount of pesticide residue allowed to be higher.
Am on mobile please excuse typos
Town is not quite a mile away. This seems like a case of "authority figures" not being able to back down.