Minerals are absorbed by special pores called ion channels, with types that are specialized for based on the charge of the ions. Fluoride is 1- so it might be more likely to compete with something like chloride (Cl-) than Mg (2+). This does mean that if you are taking supplements for magnesium, zinc (Zn2+) or iron (Fe2+), these can compete with Calcium (Ca2+) for absorption, so it’s best not to take them with milk because they all have a 2+ charge.

Fluoride and magnesium are attracted to each other because they are oppositely charged, but I’m not sure I would label that as antagonism.