Getting better at code is something you can verify automatically, same for diff formats and custom response formats. Instruction following is also either automatically verifiable, or can be verified via LLM as a judge.
I strongly suspect that this model is a GPT-4.5 (or GPT-5???) distill, with the traditional pretrain -> SFT -> RLHF pipeline augmented with an RLVR stage, as described in Lambert et al[1], and a bunch of boring technical infrastructure improvements sprinkled on top.