Thiomersal is very toxic by inhalation, ingestion, and in contact with skin (EC hazard symbol T+), with a danger of cumulative effects. It is also very toxic to aquatic organisms and may cause long-term adverse effects in aquatic environments (EC hazard symbol N).[21] In the body, it is metabolized or degraded to ethylmercury (C2H5Hg+) and thiosalicylate.[15]

https://en.m.wikipedia.org/wiki/Thiomersal

The toxicity of ethylmercury is well studied.[4][1] Like methylmercury, ethylmercury distributes to all body tissues, crossing the blood–brain barrier and the placental barrier, and ethylmercury also moves freely throughout the body.[5]

https://en.m.wikipedia.org/wiki/Ethylmercury

They don't try very hard to make their case, they rely on the general disinterest of the public.