Sorry I might not have been clear. There are two different extensions available; the one you are currently using adds a button to the heatmap webpage where you copy the URL into custom imagery. There is a new and improved extension that bypasses this step and patches iD such that you can add Strava imagery as an overlay rather than a custom layer. That way you can overlay it onto other background layers. It also doesn't leak your Strava cookies in the changeset tags 😉 