(I work at Google, on Chrome)

I know both people from both the MSE team and the Project Stream team. Project Stream doesn't use MSE (it uses WebRTC) and the code in Chrome to detect live streams and optimizing them has been around long time, (see, for example, this change from almost 4 years ago: https://codereview.chromium.org/692323002) and they've steadily added better support for live streams over the years. It's not "game-streaming tech in a web browser". It's just a few simple heuristics to function better for low-latency streams. You can read the code for yourself if you'd like:

https://cs.chromium.org/chromium/src/media/renderers/video_r...