On LessWrong, Bayesianism is probability theory. Moreover, it is bundled in with subjectivism about probability, determinism, many worlds theory etc. It all comes down .to whether the OP wants to become a rationalist, or a LessWrong rationalist, like deciding whether you want to be an economist or an Austrian school economist. If the former, some unlearning will be required.
I don't think there a fixed concept of what a "LessWrong rationalist" happens to be. LW is fairly diverse.
The current state of my understanding (briefly):