I just want to leave a link to your previous post:
In case anyone can use those findings as inspiration to create a better solution, or become enlightened to how they may include WebP images in their own project.
It was determined that the WebP format can be displayed in AutoHotkey using external DLL files. The DLL files can also be encoded into the script file so that there are no requirements to include separate files.
Built-in support would be great, however.