This whole thread feels like one of those "I can tell the difference between an MP3 encoded at 320 kbit/s and one encoded at 256 kbit/s!" audiophile threads. Yes, there are probably people out there with well-calibrated ears who can, but I am sure not one of them. FWIW I have a 27" 5k iMac and can't even remotely see any difference between the images.
There is definitely something changing though, because if I open each in Preview, switch Preview to full screen, set the view to be actual size, and take a full screen screenshot, the screenshot for the WebP image is 14% smaller than the one for the JPEG.
If I use screen zoom to go way in and then flip between the two images I can finally see some changes. The JPEG background has more small scale variation in shade. In the hair there are some white streaks that aren't quite as long in the WebP. Lots of small changes in the shirt, but it is about 50/50 whether or not any given difference there looks better in the JPEG or the WebP.