Bacteria is needed for composting to happen. Milk with antibiotics in it would hurt that process.
Disclaimer: I'm not a farmer, but I do manage the IT/automation of my/a family farm.
But below a certain concentration some antibiotics won't have an effect hence, no evolutionary pressure
If at the concentration presented it kills/affects some bacteria, yes, what you mention happens, otherwise it doesn't.
Of course, I may be wrong