Cowpox was used to vaccinate against smallpox. Cowpox is a less severe infection that stimulates the same immune response as smallpox, but it’s not the same virus.
Inoculation is where a weakened pathogen is introduced to create immune resistance against that same pathogen (see inoculation parties).
MRNA is not weakened virus, it's a spike protein in this case.
Or so that's the argument.