I think this comparison was probably because the viral polymerases encoded by coronaviruses are less prone per cycle to replication errors that cause mutations than some other viruses, such as influenza viruses. So for each instance at a molecular level of a polymerase replicating an mRNA sequence, a mutation is more likely for influenza viruses than for coronaviruses.

At the same time, an extremely infective virus like SARS-CoV-2 (especially with newer variants) will produce so much viral load that there becomes more opportunity for mutations per infection. Combine that with far more individuals being infected with SARS-CoV-2 each year than influenza, and you have SARS-CoV-2 mutating at a much faster rate.

Stopping the spread of the virus would be the most effective strategy to stop the continued development of new mutations.