Yes, it is a mistranslation.
No, it shouldn't be "transmission".
"réducteur" is the word. Translated as a noun (gearbox) by Google, it's in fact an adjective (in this case), meaning the leaks reduce the amount of sense the BEA put on the accident.
I'm not confident enough on my english skills, but perhaps 'reductive' or 'reductionist view' should do?
The BEA is saying it's too simple (and unrepresentative) to (just) say "pilot error, and that's all folks" (which is ~ what the mass media are doing, they don't want a complex answer, it has to be the plane's fault, or the crew's fault, but keep it simple for underbrains, please).