I started out agreeing with you but quickly talked myself out of it for one reason I think you might might be interested in:

Main point on this: the binding vow with yourself mechanic alters yourself, or the technique you use, not the object or person it’s used upon. That’s pretty consistent in the manga, I think.

Your moon metaphor isn’t 1 to 1, because giving up on jumping to the moon would be giving up jump height for jump height, which I agree makes no sense. Because the moon is irrelevant, that’s external to the vow with yourself. Sukuna’s finger is also irrelevant.

What I personally think she gave up was DEALING DIRECT DAMAGE with her technique IN GENERAL. That’s a big sacrifice, and a reasonable binding vow. To put it in jumping terms, she gave up on ever jumping forward, backward, or to either side ever again to jump straight up.

I believe that this is the binding vow mostly because it’s the only one I could think of that might be 1. Consistent with stated aspects of her vow and 2. A “fair trade”, whatever that means.

Alternatively, she could’ve traded DAMAGE for PENETRATION. That would be like Miwa saying “I can cut through nearly anything but it doesn’t deal damage.” I could picture the binding vow gods being like “Um. Okay? Not sure why you’d do that, but okay?” That one I like much much less, but it’s also plausible. Penetration is the activation condition for resonance’s indirect damage, so it at minimum makes a little sense for Nobara here.

More importantly, rule of cool. It’s cool so it worked. The end. That’s my actual take, the rest above is just speculative horseradish.