You certainly can run CPU heavy things in a Tokio runtime, but it generally only makes sense if you only use the runtime for CPU heavy things. Putting any network IO on the same runtime would be a pretty bad idea, since your IO would frequently end up pausing for a long time. At this point I usually ask why you wouldn't just use rayon instead, considering that rayon also provides one thread per CPU core, but it has become clear that there is demand for using Tokio in this way, even if I don't really understand why.
Regarding spawn_blocking, I actually point out that it is unsuited for CPU heavy stuff in my article on blocking. It is designed for blocking IO.