The 30-40 year figure assumes the related virus is a direct ancestor and it stayed within the same species, which is quite a big if. It's useful as a metric within a single population, but not exactly evidence hard enough to play genetic detective.
If they just share ancestors that time is basically halved towards the most recent common ancestor, which puts it back somewhere in the mid 2000s. When evolving in parallel, within different species, the divergence grows really quick. Also when viruses jump species the mutation rate skyrockets at the beginning[0][1] to adapt to the novel host, which could easily account for most of the difference between RaTG13 and Wu-1 anyway.