I am a liver transplant and take drugs to reduce my immune system to avoid rejection.. Studies showed that people with solid organ transplant and similar conditions did not reach the same level of protection as "normal" people with two shots. This "third shot" was given to to this specific group of people to help them reach that higher level of protection. It is not a "booster" per se.
Coronavirus (COVID-19) Update: FDA Authorizes Additional Vaccine Dose for Certain Immunocompromised Individuals | FDA
"Booster" shots (of which only the Pfizer is currently approved) are the result of normal healthy people showing a reduced response after 6 months. This is an entirely different condition than the above action taken in August.