His goal could simply be to learn SOTA architectures.

When rumors started that GPT-4 design would be kept secret, he likely wanted to know what architecture it would be. Perhaps he left Tesla, waited out the non-compete clause, and joined OpenAI to learn its details.

When Mythos dropped, there were hints that it had a new architecture. He might similarly want to know how it works.

Either way, there is enough cross-lab hiring that those secrets eventually get known, but only by the labs.