Hi Mariana,
This is one of the situations where not the full story is given here. Due to its mode of beta decay in 131-I, the isotope is much more dangerous to human health than 123-I, despite 123-I engaging in more gamma radiation (of which apparently it has a VERY short life span). The answer would be arrived at by information in the passage, where it specifically states that "90% of the radiative damage is caused by emitted beta particles" (in paragraph with 131-I.
Hope that helps!