add_image_size() width not actually working

After playing around a little more, the image does look like it’s
being resized by wordpress on upload to the 708 width, however, when
inserted by the media library, the image is automatically scaled down
to 474px which is the width of the text column.

This is correct. Themes define a global variable called “$content_width” which tells WordPress the width of the main displayable content area. WordPress will automatically put images into a container with this width, so as to not overflow the content area.

The resizing of images on upload is independent of this process. The image will be whatever sizes are defined for it, but it will scale into the space provided for it by the content width.