I’m not sure if there is just some “point of diminishing returns” or whatever where JPG actually becomes more efficient or what.

There is, but it's at high quality levels. If you're using WebP for thumbnails or other lower quality situations (Which was the original intended use) then WebP will give you better quality than JPEG for a given filesize.

For lossless uses it's even better, the format is much more limited than PNG, but in the common cases it beats it.