All they did was say that 2020 doesn't count against your eligibility. So if you remove 2020, he played 2021, 2022, 2023, and 2024 - that's his 4 years of eligibility. He doesn't get to RS 2020 and then get another year.
Cook has said this was his last year:
https://collegesportswire.usatoday.com/2023/12/30/missouri-tigers-brady-cook-quarterback-return-2024/