Accoring to the proposal (https://wiki.apache.org/incubator/BeamProposal), OSS impl. of streaming is on the way, by Apache Flink, etc.
The blog is just suggesting the model itself is superior, regardless of OSS or not.
But assuming it's true. it's very welcome news! I'll be keeping a close eye on future releases.
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).