It's not. Check the statistics in Hong Kong, where hardly anyone died of they were boosted with the BioNTech vaccine, vs almost all the deaths occurring in unvaccinated or Sinovac vaccinated people.
What you're claiming is easily disproven, just by looking at the statistics.[1]
In Hong Kong, 3 doses of Sinovac or BioNTech came out almost exactly even in preventing death. In fact, Sinovac turned out to be slightly more effective, though that could be statistical noise.