Just on its surface I find it extremely hard to believe sending a small HTTP GET request to the advertisers webserver and pulling a .webp is anywhere near as energy consuming as:

1. Arranging for the billboard to be built

2. Hiring workers to drive to the site and build it

3. Finding an advertiser to put on the billboard

4. Printing their advert on a canvas

5. Hiring someone to go hang it up

Plus every time a worker needs to drive to the site to perform maintenance.