Honest question: if I can't see the artifacts without zooming in, is there any reason why I should care? Of all the conversion results offered, the Photoshop export actually looks the best to my eyes, so I'm genuinely trying to understand why the jpeg.io result would be considered better quality in this particular case.

I don't need to zoom in or even look that hard to see the artifacts. Maybe that's me, or maybe that's the retina display on my MacBook, but that's why jpeg.io looks the best for me. If you can't see the artifacts though, I would say jpeg.io loses out.

For me, it looks like a different image. As in, I would never have expected that the differences to the original weren't a (bad) choice by the "designer" making the image. That seems risky, and in many use cases a worse kind of artifact.