But it's also important to note that only $300K of that is in cash. The other $600K is in profit participation, which could take years (maybe even a decade!) to be realized. It could also be worth $6M a year when it's realized.
But ultimately it's an investment of your time. Or to think of it another way, you're getting paid $900K a year but you're also investing $600K a year in OpenAI, which may end up being an amazing return or nothing at all, just like any startup investment.
Although with Sam at the helm, my guess is it will probably be worth more than $600K a year.