Harry Potter would have posted the original video to YouTube instead. It seems like a tragic irony, and perhaps a sign of danger, that OP posted the demo video to the website of He Who Should Not Be Named. Why? Why?!
Because Twitter posts consisting of YouTube links die quickly. It’s very obvious once you have a few thousand followers.
Your option is basically either upload to twitter, or put the YouTube link at the end just before a screenshot. Or both a video and a YouTube link, I suppose.
If you trigger their YouTube embed, it seems like it gets penalized quite harshly. I’ve seen other people agree with the sentiment.
Just don't use a Twitter post as your demo video in the GitHub readme. I don't care what someone does on Twitter - I never go there. But you can just embed a .gif or .webm in your readme or link to YouTube there.
I'm in the US and twitter videos don't play for me. I see what looks like a video control when I view the post, when I click it, it throws up a modal log in/sign in dialog. No way to view the video while logged out.