Fork() is now basically the root of a looong list of special cases in so many aspects of programming. Things get even worse when you use a language with built-in runtime such as Golang for which multi-threaded programming the default behaviour. If fork() can't even handle multiple threads, what is the real point of having it when a 8 core 16 threads AMD processor is about $150 each.
These threads and those threads are not the same. The 16-threads SMT processor will happily chew on 16 different programs, processes or whatever the load at the moment is, e.g. if you use Python's multiprocessing you can create 16 processes and they'll be executed in parallel.
fork() can handle multiple threads but you have to be attentive when cleaning up etc. - quite often, code using fork() will get confused when you spawn threads, and code using threads will get confused when you fork()