Earlier quoted context omitted.
They are not the same thing. Threads share address space and processes don't and that's a crucial difference.
I said same thing with different flavors. Literally, they are both represented by the same data structure in the kernel. The salient point of the discussion - cost of forking a process versus cost of spawning a thread - is the same. They require different styles of programming, but I was responding to your point on cost. If you ever need to reason about scheduling in the Linux kernel, then you need to understand this…
I am not familiar with current (less than 10 year-old) processor architectures, but, back when I was familiar with that stuff, switching context between threads of the same process was a lot less costly than switching between different processes because the memory context is the same and set-up and tear-down was a somewhat costly operation. As processors start getting more cores, it also allows schedulers to keep threads of the same process to the same cores, reducing context-switch overhead (or to power-down less used cores to conserve energy).
If I had a very good threading/multi-core support, I would probably go with Solaris on a 64-thread T2 SPARC thingie, not Linux on 6-thread x86. And that's a good reason for Sun to invest some money on helping convince Guido the GIL is bad and will become worse with time. Because Intel will probably catch up.