Himiko has been in the news and I’d like to be sure I have the context of this correct. They can tell the light is 12.9 billion years old by how much it’s red shifted. That’s roughly 800 million years after the big bang. In that time, the universe has expanded by a factor of 8. If the light waves stretch by that factor, that pushes visible light into mid or maybe even far infrared part of the EM spectrum. Himiko is a Lyman-Alpha Blob. They can tell this by the emission lines that show up in a spectrometer. Lyman-Alpha Blobs emit very specific UV light. Us earthbound observers have a tough time seeing that through our atmosphere. Luckily, it’s so redshifted that the lines end up in the visible or infrared parts of the spectrum. The distance between us and Himiko is now 8 times greater than when that light started its journey. We started off much closer than 12.9 billion light-years apart and we are now much farther apart. So the angular size of Himiko could be misleading. What I mean is Himiko probably looks much closer than you might guess from the age of its light. It looks like it’s only 4 billion light-years away. And that’s how far it actually was when the light left Himiko.