I don't think they're doing chunked encoding. I think they're making their template engine emit a stream instead of emitting the rendered page. Then, each time the template system has some text to send, it's written to the network rather than accumulated in a buffer that's all written at once.
The innovation is not what headers to send, it's to produce your data incrementally instead of all at once.
What I meant to write was that I don't think the encoding is what matters; incremental writes to tcp sockets are easy, and http is not much harder. What's hard is making your application code work well with this.
The innovation is not what headers to send, it's to produce your data incrementally instead of all at once.