I did the same, and it took me a long time to spot it, but in the upper-right corner you see circles in the WebP version. It's outside the centre of attention, so it's not that obvious. Actually, it wasn't until I saw the second picture and knew what to look for that I spotted this in the first picture.
It's not so easy to see if the browser zooms the image, so make sure to open the image and set zoom to 100%. I also need to keep my face fairly close to my screen (12" 1920×1080, so not that large).
That said, in the context of showing off your photography I can understand considering these kind of artifacts undesirable, even though they're perfectly fine for a lot of other uses. On my own website I spent quite some time downgrading my mugshot to be as small as possible without too many artifacts – it's now 4.9K in WebP, vs. 9.2K in JPEG before. Maybe that was a tad obsessive though...
I do think the author doesn't quite appreciate that most people are not photographers, and that for most images quality doesn't actually matter all that much.
It's not so easy to see if the browser zooms the image, so make sure to open the image and set zoom to 100%. I also need to keep my face fairly close to my screen (12" 1920×1080, so not that large).