Looking at the ingredients list on Wonderbread white bread, could you make that at home?

You can make bread with salt, flour, yeast, and water. Most breads in the grocery store, however, have considerably more ingredients, which are more in the purpose of treating the foodstuff as an industrial product rather than for nutritional purposes.

(That's not automatically bad btw. The amount of ultraprocessed food you can eat is actually probably quite a lot in relative terms before it starts causing health problems --- the problem is when it becomes 70-80% of your diet.)

He's talking about "wonder bread" and other factory breads that have had much of their nutrients stripped and some put back, to the detriment of their absorption. Some also are concerned with artificially included preservatives and the unknown unknowns of putting them in places (even if there's a common natural source in another food).

Homemade bread is certainly not ultraprocessed (especially if made with unbleached flour or even better, whole wheat flour), but factory bread most certainly is considered ultraprocessed.