I'm guessing the reason was that this was born out of a hackathon. E-greedy is likely the easiest algorithm to implement for a MAB problem like this. Given that

I'm guessing the reason was that this was born out of a hackathon. E-greedy is likely the easiest algorithm to implement for a MAB problem like this. Given that they mention there's an API to specify sort order (assuming it was on a per-view basis), the specific algorithm could be swapped out for something smarter. I think a contextual bandit algorithm would be interesting in this case, although I wonder if there would be enough data behind photo swipes to back each context meaningfully.