This is a fair question. It is based on the chemistry of fluorine.
Fluorine has the strongest electronegativity of any element in the periodic table. It requires extreme measures to muscle fluorine off a molecule. This is why it is used in non-stick surfaces like Teflon (nothing can “grip” the surface molecules because fluorine won’t let it) and why it is used in toothpaste (molecules that might attack the tooth surface chemically can’t compete with the fluoride that is already there). The dark side of this is that it is difficult to contain fluorine compounds, they have a tendency to attack most containers you can put them in that aren’t also fluorine based. Famously, they tend to eat glass so you can’t store it in glass vessels.
The toxicity of fluorine flows from this. It has an insatiable appetite for Type II metals, notably calcium and magnesium in the human body. If you are exposed to fluorine, it will have a seek-and-destroy mission for these metal ions. A typical human body has a lot of calcium and magnesium circulating so it can absorb exposure from diet, water, etc. The net effect is that some calcium and magnesium is removed from circulation and is no longer bio-available. Not a big deal. In extreme exposure cases, like an industrial accident, the way it kills you isn’t toxicity per se but by removing all of the calcium ions from your system. Your heart uses calcium ions for electrical signaling, so if those are all neutralized by ravenous fluorine, your heart stops.
The antidote for extreme fluorine exposure is to ingest a bunch of simple calcium and magnesium salts. The fluorine latches on to the surplus floating around and there is enough left for your heart to keep running.
This is where the mechanism of action question comes in. For fluorine to have biological effects on cognition, the body would have to be so devoid of neutralizing calcium and magnesium ions, which it strongly prefers as a matter of physics, that you’d already be dead. In extreme exposure cases (like getting concentrated fluorine compound spilled on you) with prophylactic calcium/magnesium antidote, it does really nasty damage to the bones, but there has never been a case of cognitive damage that I’ve ever seen mentioned in the safety literature.
Fluorine is a nasty element, I don’t miss working with it, but it isn’t a serious threat in trace quantities because human bodies can easily absorb the loss of calcium and magnesium. Human bodies are tolerant of almost all elemental toxins at natural levels. The few for which there is no evidence of tolerance at even trace levels are elements like mercury. Even elements like arsenic and lead are believed to be required by human biology to some extent and therefore the human body has some evolved tolerance for them. (These two are pub quiz material, most people are shocked to find out that these are necessary micronutrients.)