I just ran into the same issue
Turns out you can can call the Intensity of the hdri backdrop by calling the “material parameter track” from the HDRI backdrop Geometry element
In the sequencer editor add your hdri backdrop element, then add a track into it calling the “geometry element”, and then add a track to the geometry track calling for “material parameter”
Then you will be able to call for the intensity and animate it
Thank you for pointing me in the right direction Stef 