I have already done:
guix package -i webp-pixbuf-loader ffmpegthumbnailer
Other filetypes like png and jpg show thumbnails just fine.
I’m not sure what happened, but lately I’ve noticed that thumbnails are now finally showing except for avi files and that seems to be an issue with ffmpegthumbnailer itself, so I’m going to attribute this to older versions of the above two packages having been insufficient, while avi has yet to follow and mark this as solved.