We're sorry for the confusion. It looks like you're trying to use the fix elements option in XD. The option has been moved to the Prototype tab. Once you will select the object in your artboard the Fix position when scrolling option will be highlighted.
Please see the attached screenshot file.
Thank you for sharing the video link, I will make sure to forward the link to our concerned team to get it updated. Please let us know if you face any issues with this. We'd be happy to help. We're sorry for the inconvenience.