I think it was mostly a bluff to try the pressure the board. I don't think Sam and most of Open AI rank and file would want to be employees of MSFT
(what isn't)
Let's say Sam called his broker and said to him on Friday we'll before the market closes. Buy MSFT stock. Then he made his announcement on Sunday and on Monday he told his broker to sell that stock before he announced he's actually coming back to (not at all)OpenAI. That would be illegal insider trading.
If he never calls his broker/his friends/his mom to buy/sell stock there's nothing illegal.