It's pretty easy to make desks out of doors. Especially if you don't need drawer or rackspace. I read somewhere that they got the doors as salvage from a waste disposal firm. Putting a few screws here and there would have sufficed. Not to mention door-length desks would be really spacious, for leg space and desk space.

We all use automatic standing desks now, fwiw. Allegedly for period the door-style desks were actually more expensive than normal desks but they were kept as a "cultural statement". Don't know how true that is.

This was corporate myth-making. They did it to drive home the point that Amazon was a scrappy place. Obviously it wasn't a logical decision from a pure cost basis. But from a story-telling perspective it was a heckin' home run.