I assume that the speaker means that the anti-doping laboratories' concentration was estimated at 400 x 50 = 20,000, but one cannot be certain. S/he might meaning that the reduction was 20,000, so the anti-doping laboratories' concentration was 20,040. Such expressions lack clarity.
I have seen similar confusion with: We had 50 enquiries today, 100% less than yesterday. I assume that the speaker meant that there were 100 enquiries yesterday, though that would seem to me to be 50% less (or fewer!).
In my own speaking and writing I try to avoid using such expressions unless I can be absolutely clear, as in: The January sales of £120m were 20% up on the December figures of £100m, the biggest percentage increase for eleven months.
Note that in my example I put in the actual figures for January and the word percentage. It may well be that last July's figure's were £500m and August's were £550m. This is a a bigger increas (£50m as against £20m), but a smaller percentage (10% as against 20%).
My only suggestion is to take verbal descriptions of figures using percentages or such expressions as times, more than, less than, etc with a large pinch of salt. If you can, ask the writer/reader to give you the actual figure.