Is that really true? I know it's what I've been told for 20+ years but ...
Iodine kills in a couple ways. It's in a hyperosmolar solution that sucks water out of cells and kills by dessication ("dries" out bacteria). Halogens also have some effects on DNA synthesis and direct oxidative effects. Iodine specifically enters cells and binds to proteins and nucleotides causing destruction and death. All of this takes time, which passes while we sit around waiting for it to dry.
But none of this relies on on the vehicle it arrives in being "not wet" ...