This is true to some extent, fluoride though tends to be residual in bones and teeth, and other tissues, Iodine tends to be easily displaced, and once tissue has Iodine saturation, the excess is excreted, predominantly in the urine, so it is very unlikely that a person could build up a compounding excess.
I do agree that too much of anything is never good, as the old saying goes, all things in moderation, even moderation.