It's not 100% clear these are that different from RTGs it sounds like they just use Stirling engines instead of thermocouples to do the heat -> electricity conversion which is more efficient than the old style but seem largely the same otherwise.
https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_ge...
Kilopower will utilize highly-enriched uranium fuel to power a fission reactor. This means that the atoms in the fuel will be sustaining a chain reaction of fissioning, releasing neutrons, causing more atoms to fission, and on and on. This is what generates heat in reactor systems. This design will prove usable for shorter, higher energy demand missions.
https://www.nasa.gov/directorates/spacetech/kilopower
An RTG is essentially a lump of fuel which sits and decays, generating heat. Kilopower's reactor would be an actual reactor, needing to manage reactivity, burnup, and maintain criticality. They are vastly different systems. For someone not in the field I can see how there'd be some confusion but please be less speculative on topics of which you're less familiar.