That's too subtle for me to worry about. I'll just go with what sounds right (based on what sounds right in English) and I think I'll be okay.
::edit:: That website is also filled with grammatical errors. If you can find something that provides examples rather than confusing explanations, please let me know. (I'm searching and failing. I don't blame you for picking that website.)
::edit2:: I found this explanation more helpful: http://community.oxforddictionaries.com/boards/topic/between-vs-in-between/
Until now, this was one of those things that I just did automatically and never had to think about. Never had an ESL student ask me about it either.