Mathematically 600% less energy is impossible. You can only...

Mathematically 600% less energy is impossible. You can only reduce something 100% eg 1 reduced to zero is a 100% reduction or in other words the thing reduced is reduced to nothing.

I think the researchers mean they reduced the power required by 85.71% which is the same as 6/7
For example if your starting point is using 7 units of electricity and you reduced that to 1 unit used you have reduced electricity by 85.71%. I think the researchers are assuming the reduced number of units used being 6 is a 600% reduction versus what was used being 1 unit.

No matter what, this looks like a good result for the researchers, they just got the headline wrong.