You're right, ice core data only goes back 800,000 years.
But there are other methods.
https://today.tamu.edu/2021/06/14/ancient-deepsea-shells-reveal-66-million-years-of-carbon-dioxide-levels/
Space/Science » in reply to Hey, I didn't make the chart.
But there are other methods.
https://today.tamu.edu/2021/06/14/ancient-deepsea-shells-reveal-66-million-years-of-carbon-dioxide-levels/