Is the Dynamic midrolls and pre rolls part of the hls spec?
Dynamic mid-rolls in HLS
"HLS Interstitials" or just "Interstitials" is how we tend to refer to it. At the moment, it has not been added to the spec. There is just the preliminary specification ("Getting Started with HLS Interstitials") that the WWDC talk ("Explore dynamic pre-rolls and mid-rolls in HLS") references.
The formal specification will come after we've all had a little more experience with it. This is similar to the way we introduced Low Latency HLS.