Polonium decays at the same rate whether it’s on clothes or in the body. It does not magically stop decaying to lead because it was taken as a sample or the clothing was removed from the body.
So if a single polonium atom was detected, then that suggests that at the time there was a live Arafat, there were a million of them in that sample. OTOH, ppm trace emlement tests are nothing new, especially for extreme elements like the radioactive ones. However, if the concentration is too high today, then it would imply that Arafat should have blown up like giant red beet and lost all his hair while vomiting blood, etc. I.e. too large an implied dose means the material was planted much later.