Perhaps it follows the Hollywood model. There are lots of features in development, staff-shipped, in discussion, in the backlog, or whatever. Then there is some external stimulus (Facebook launched reactions!) and the feature gets quickly tailored and green-lighted.
So there is a quick reaction, but work on the idea may have been hidden from the public for a long time.
Version 3.4.0 of Octicons [0] came out on January 22, 2016. It added "smiley" icon, which was used in this feature today. That's 8 days after the dear github letter. 7 weeks before today.
That's a bit suspicious, really. A feature like "Reactions to a comment" doesn't take two years to implement, even with all the scaling considerations. And people have been complaining about that for at least two years.
So there is a quick reaction, but work on the idea may have been hidden from the public for a long time.