They changed the definition because the technology changed. The word 'trolley' used to mean cart but it doesn't any more because the technology changed. mRNA technology is over 50 years old but it only recently has been applied to vaccines, so that definition had to be changed.
You're looking for hidden meanings when there are none.
"Immunization: The process of making you immune or resistant to an infectious disease, typically via vaccination"
https://www.verywellhealth.com/the-difference-between-immunization-and-vaccination-4140251
They changed the definition because the technology changed. The word 'trolley' used to mean cart but it doesn't any more because the technology changed. mRNA technology is over 50 years old but it only recently has been applied to vaccines, so that definition had to be changed.
Get a grip.