Unfortunately, the given URL is not working for us and it redirects to a page telling that the link is expired. I pasted the code you mentioned in the original post to the Raw Content element in Cornerstone and the video in POP up screen is responsive.
But I tested the case on the theme version 5.2.4 and Cornerstone version 2.1.6. So if it is not responsive in your case kindly update the theme and Cornerstone to the latest versions.
Unfortunately, you can not use the _scale property for the videos, it is valid only for images. The code you use is an iFrame which is getting in responsive mode automatically and the -scale property cannot change that.
Thank you for your understanding.