I wondered this too, but I cant say anything about how they got the sensor. I just came across the webside by pure chance - I was simply googling for large format, and there it was. Cant find any secondardy source about this project yet, and cant find anything about how they actually produced this sensor on the website itself.
For all I know, theres been speculation going on to change the way how sensors are produced for really large formats. Right now what the companies do for any chip is using an huge and extremely expensive lens to project an image on the chip, which has been previously treated with photosensitive materials, then work on the resulting structure. One could however also use a more scanner like technology, so instead of producing the full image, one moves over the chip with a scanner line, only always projecting a small part of the whole structure on the photosensitive material. This might be beneficial for really large sensors and result in much smaller cost for such a production machine, and making it feasible to produce larger sensors in one piece. The production itself would be slower this way, of course, but thats not so important since the demand for medium format sensors is also quite low.