SQL would certainly have been an adequate solution for this. In our case (I work at Artsy and contributed to the code described in the post), our environment relies primarily on MongoDB already.

I think the work is interesting precisely because it achieves some of the benefits of transactional systems using a data store that isn't usually considered for these purposes. Immutable documents and serializing the processing of bids (to limit concurrent access), as described in the post, helped.