The ergonomic idea behind it is that during typing, you have the controls "right at your fingertips" (as Apple likes to describe it). If you have your fingers all on the keyboard, then it can absolutely be more ergonomic to just reach up 1cm with one finger to perform an action and continue typing or whatever you're doing, instead of having to move your entire hand down to the trackpad, then move the cursor to the respective button on-screen (which can oftentimes be hidden in the menubar or some submenu, requiring multiple clicks and cursor movements to even get there), press the button, potentially move the mouse back/out of the way and finally move your entire hand back onto the keyboard before you're able to continue.
Comparing these two is, by definition, an ergonomic question, you are comparing two arrangements of control schemes and ask which one allows to do the user something in a more efficient way which is what ergonomics are all about. It's not just about eye movement; how much you have to move your hands around, the distance and amount of different sub-paths that your cursor travels and the amount of clicks required to achieve an action is also a big part of ergonomics.
Now I'm not saying that the Touch Bar is ergonomic in every instance – by far not. If your hands are already on the trackpad for example, then it is usually faster and easier to move the cursor to the respective on-screen button than to move your hand up to the Touch Bar. The TB might not be an ergonomic winter wonderland, but that doesn't mean there aren't some applications that are just more ergonomic when performed on the Touch Bar than with whatever the alternative is. So universally claiming that there's nothing ergonomic about the Touch Bar is something I'd strongly dispute.
Now this I can get behind, and as I said that's something I'm also a bit cautious of right now. I think an external keyboard with Touch Bar is still something that can turn out useful, but I'm also curious how much the additional eye movement required on a desktop setup like that will negatively affect the experience, compared to a laptop.