Flink's event-time support is coming along nicely. Their first round of true event-time support came in November (https://flink.apache.org/news/2015/11/16/release-0.10.0.html), and much more is on the way. Flink will be an excellent platform for Beam, both batch and streaming.

As I understand it, Spark has event-time support coming soon as well. I think basic stuff is landing in 1.7. Not sure precisely what they have planned, but I can only imagine that Spark will also become an excellent platform for executing streaming Beam pipelines in due time. In the meantime, the streaming runner for Spark can either target those features which Spark does support well (i.e., processing-time windowing, in this case), or try to emulate those it doesn't (such as how it was done in the article).