Molly, best way of explaining this is to direct you to this webpage and asking you to read the section called 'How Your Body Decides How Much Milk to Make'.
This notes that the early days are a time when the body calibrates the mother's potential milk supply by laying down prolactin receptors, and the more the baby feeds, the more prolactin receptors there are. If people want to know more, there is quite a literature on the prolactin receptor theory (theory in the sense of 'coherent explanation', not in the sense of 'only' a theory as if someone is guessing :)).
The risk with early formula bottles is that milk supply is calibrated downwards - this may not matter or even be apparent for some time, given that often, nature 'makes' women actually over produce at first, which may compensate. But the risk is that later, when that initial over-production ceases, supply may not be enough.
Again, not necessarily something that is apparent or something that matters - because the baby's intake (and supply) can often be ratcheted up again by feeding the baby more often.
But because nature does not 'expect' the new baby to have any other food than human milk (and for 99.99 per cent of human existance, this would have been a hazardous thing to do), anything that does interfere with this, is by its very nature, 'undermining' to the physiological process.
This is not the same as saying you, or anyone else, does an unjustifiable, or terrible or risky or automatically doom-laden thing by giving formula. Maybe it will have no apparent effects on the baby or on the milk supply.
But it's not a physiological thing to do - hope you understand I am not judging but explaining :)