If I understand right what @Hudson555x was suggesting here in his post in the thread he linked to, you would be cropping the black borders of the 9:16 video so that it would fit in a 16:9 video. You would not be cropping out any of the image.
So if you have the video mode in Settings set to Automatic, you import your 16:9 video first which would set the aspect ratio and resolution for the whole project the same as your 16:9 video is in. Then you import the 9:16 video, apply the crop filter to the 9:16 video to crop out the left and right black borders. By doing that it will allow the video in the center to settle in the 16:9 space.