Re: So....how was it really tested?
How have they cheated - the standard supply voltage to an in-line amplifier (ILA) is 50 volts, the only reason high voltage is used in submarine cables is because the ILAs are connected in series on the power conductor.
Our office lab has quite a few spools of fibre mounted up in the ceiling space, a mixture of 50, 100 and 200km spools - they are not very big, about 2/3rds the size of a box of cat5. The size difference between a production cable can be handled, laid on the ocean floor up to 10km down, left there to work for 20 years or more, and a spool of fibre designed to be installed in a lab is, strangely enough, hugely different. The naked fibre wound on to a lab spool is thin, so thin it can be hard to see when looking at a single strand. You could, literally, fit millions of km of naked fibre into a single cable tank on a ship.
Building up a set of spools to 12,000km is going to cost a bit, will weigh a lot and will take up a good sized corner of your office, but if you have the budget it's a trivial thing to do. How the DC power gets to any ILAs is irrelevant - you design the production cable insulation to meet the power feed voltage needed for the span at 50 volts/ILA, but that has nothing to do with its optical characteristics.
It's normal practice when testing a system in the factory to rack up the ILAs and terminals side by side and interconnect them with spools connected in series to provide however many km of fibre each span is designed for - no cheating and a lot more convenient than having a factory hundreds of km long and spending half the day driving between racks. (I've spent more than enough time driving up and down a system, taking two weeks to resolve a supervisory channel problem that could have been sorted in the factory in a day or so with everything side by side).
THEREFORE - based on many years of working with long haul high capacity fibre systems and quite a few years of working for a manufacturer of both terrestrial and submarine systems (including time at the factory observing pre-delivery tests on behalf of a customer), I'm going to call no BS on this one.