"Our first finding was that requirements 2 and 3 are satisfied by a feature called Message Groups. What came as a surprise...ActiveMQ and RabbitMQ, do not support this feature"
Actually this is right, so the information in the article is somewhat incorrect. Thanks for spotting this!
There is however an issue with the ActiveMQ implementation: once a user is assigned to a consumer, it is hard to make ActiveMQ rebalance it. At least by default the load-balancing does not take place until a consumer is dead. In case of a chat bot, some users may be returning, while others may quickly go away, leading to drastic imbalances. Apollo does the balancing perfectly out-of-the-box.
A quick search seems to indicate that ActiveMQ does support message groups: http://activemq.apache.org/message-groups.html