As Jonathan mentioned, we made this decision around 9 months back and at that time Kinesis wasn't as mature and had less flexibility around retention period etc.
Kafka is very reliable (as I had seen it handling billions of events a day at LinkedIn) and has a huge open-source community around it. At IFTTT, we always prefer to use and contribute to open source ( http://engineering.ifttt.com/oss/2015/07/23/open-source/ ).