Breastfeeding is the biological norm.
You can't say that breastfed babies have 'extra' protection from breastmilk unless you accept that formula feeding is the norm against which you're comparing it.
Medical comparisons don't use the social norm as the base against which to measure the efficacy of an intervention, they use the physiological norm.
Breastfeeding is not 'best' or 'the gold standard' - it's just the biologically normal way to feed a baby.
If formula feeding doesn't measure up to this basic standard then it's fair enough to call it 'deficient' or 'incomplete'.
In some cases the harm it causes is measurable (such as in higher blood pressure and arterial stiffness in adolescents ff as babies), but usually it isn't. This is true for many things - not just infant feeding.
There are dozens of children running around in every school playground who appear to be completely healthy who are being fed diabolical diets at home, full of salt, fat and sugar and lacking in fresh fruit and vegetables. Just because we can't see how these things affect individual children doesn't mean they don't.
I personally can't understand how adults can accept the logic of this in relation to other aspects of children's diets and refuse to acknowledge this reasoning in relation to infant feeding.