There is an enormous difference between processes and threads in Linux and it's a fairly simple one - each process is allocated a memory space. All those mappings and page table entries are what makes process spawn expensive.

There is a terminology issue though, you're right, and that comes with using OS terms for userspace scheduling. Go coming up with their own term - goroutines - significantly simplifies the conversations. Gorountines, Erlang processes, green threads and fibers are all fundmentally the same thing - M userspace 'tasks', scheduled on to N operating system threads of execution, likely in one OS process. There are some language details as to how they are presented to the user.