Adobe Captivate Responsive resizing for a video

I think you want to leave the video unlocked from any fluid boxes and apply the following settings in your Position panel. Select the check box for Align center horizontally on the slide and change the Width and Height to pixels. For example, in a project that is 1024 x 627, I changed a video to pixels and I got 1024 for the width and 566 for the height. Previewing this in HTML5 would show only the center part of the video on smartphones in portrait view.

I am not sure to understand completely and for that reason my answer may be off topic. Do you mean something like I explained in this blog post:

http://blog.lilybiri.com/fluid-boxes-and-interactive-video-position-properties-panel

It is also valid for a non interactive video inserted as slide video. In a fluid box only event video is allowed and there is very little control over that type of video.

Am I understanding you correctly that you DO NOT want to preserve the video's aspect ratio (Height to Width ratio) but instead want to have the video distort, eg squash down or squash sideways?

On mobile devices videos will always usually play full screen while still preserving aspect ratio.