“I could be completely wrong” Well, uh, … The forward rate is absolutely determined by the interest rates up to an arbitrage relationship. This isn’t mean reversion, but arbitrage.
“there are long-term exchange rates that tend to be mean reverting”: you probably mean rates based by the purchasing power parity. This is mostly a theoretical construct and the mean reversion (if it exists at all) takes years to assert itself. “spot rate of higher-rate currency will depreciate”: Not necessarily. The real world of FX is soooo different from the relationships described in the CFA books. For instance, people profited from the yen carry trade for years even though theory would have you believe yen will appreciate against USD and the carry trade opportunity will disappear. It did not, for many years. And then, suddenly, in September-December '08, the carry trade was unwould and yen appreciated like crazy. In general, relationships enforceable by arbitrage hold up well but even they sometimes fail. For instance, you could make good money doing vanilla cash-and-carry arbitrage of crude and crude futures in November/December but there was not enough capital and storage capacity, or arbitrage desks were shut down, I don’t know. Relationships based on theory only (purchasing power parity, uncovered arbitrage) often fail to hold.