It is unclear to me (having just re-read Linus's post) where you see the claim that an mRNA cannot do harm. He says it cannot be incorporated into the genome,

It is unclear to me (having just re-read Linus's post) where you see the claim that an mRNA cannot do harm. He says it cannot be incorporated into the genome, which is true of the Covid vaccine RNAs but not necessarily true in general (one could inject the RNA of a retrovirus). But the Covid19 vaccine mRNA simply makes the spike protein and (I'm sure they checked) does not get incorporated into the genome.