There's a difference between simply being exposed to radiation and consuming radioactive substances since they get absorbed into your organs where they deal more direct damage.
Taking this into account, I would like to know how the "same low-level radioactive effects as" claim was calculated.
Very good observation, in case of some radioactive nuclides their effect is insidiously long term as they may accumulate in body (strontium-90). Many pro-nuclear people and outlets oversimplify the effects of radioactivity. Single number comparisons are not always adequate, since there are many types of radiation, exposure and effects.
For food contamination by radioactive nuclides such as Cs-137 limits were introduced in EU regulation from 1987 and this was last expanded and numbers updated in 2016:
For Cs-137 the limit is 1000 Bq/kg, in some countries it is lower - 600 Bq/kg.
The document does not seem to indicate how the number was determined, but it seems reasonable that scientists who understand radioactivity and impact of various radionuclide ingestion had some weight in the final decision.
This paper explains how the limit values were arrive at. In short, the rule is to assume that the contaminated food is the only source of food for the most sensitive ones (children). That's why the limits seem to be very strict.
Taking this into account, I would like to know how the "same low-level radioactive effects as" claim was calculated.