This is actually not too far off from how rudimentary vaccines work.

https://en.wikipedia.org/wiki/Smallpox_vaccine

"Vaccine" is actually a pretty broad term and loses usefulness in these conversations outside of its use as a rhetorical tool. Yes MRNA vaccines are vaccines and yes they are different from other types of vaccine such as Live-attenuated vaccines, conjugate vaccines and others.

They're also by design inferior to other vaccines since they only target one specific antigen, rendering them useless very quickly against highly mutagenic viruses (like coronaviruses).

The quick adaptability of mRNA vaccines to new variants is a key advantage, not a weakness.

Their rapid development and adaptability make them probably the best type of vaccine for fighting quickly mutating viruses. This is shown by the rapidity and relative ease of development and deployment of bivalent vaccines. mRNA vaccines are one of the great technological triumphs of the American industry and science apparatus.