3.6GHz from 10 years ago and 3.6GHz from today aren't the same though. Modern processors have way more specialized hardware for common tasks that weren't common a decade ago. Yes, the compute cores are going at the same speed. But you aren't getting the significant advantage of other hardware optimizations. Just look at the M1 benchmarks for goodness sake. (A bit unfair because it's RISC-V based, but even x86 hardwar…
I believe the point of the poster you replied to was that if you have software running at speed X it won't get particularly faster if you run the same software on a 32-core CPU. Not if the original system also ran at 3.5GHz. I.e. it doesn't get magically faster by throwing more cores at it. It could get faster by better or faster caches etc, and that's valid, but the additional cores don't magically help.
That depends on the software. Compiling C++ is basically linear with the number of cores for example.
If you mean that compiling gets faster.. then yes, with 'make -j' it will. But that you're compiling C++ has nothing to do with it, except that (as with most other modern languages) you have individual components which can be compiled in parallel. But this is hardly what the grandparent meant by automatically getting a speed boost just by executing existing software on a newer system.