Opinion: It was to make the group look more diverse.
It could have had other reasons. Maybe she really was in the group but was sick that day?
I've been in team photos where the CEO had to be 'shopped in because his absence but obvious desire to be included in the team photo. Before making such dramatic claims as making the group (just +1 out of like, what, 20?) "more diverse".
Two women, but yes, agreed - we don't have confirmation on the motives. To be clear for others, that interpretation isn't in the original article. It's only on this hacker news link title.
That is hilarious/sad but aren't most professional photographers running their photos through some editor application for post-processing?
(just commenting on the exif revelation)