It's old, but the material holds up well since it covers a lot of fundamentals
I think modern Java has better support for it, but if you've got mutable state spread throughout your application you're going to have a hard time no matter what.
How does one become a senior engineer if you don't understand concurrency?
Mutable state is most easily solved by having cpoies of everything, but then that's a tradeoff between performance and infrastructure/resource costs, but I guess that if you're in an all-Java shop that isn't much of an issue.
Best I can find as a source for now is https://www.youtube.com/watch?v=2y5Pv4yN0b0 -- I thought there was a link somewhere to Hickey himself saying this, but can't find it.