Muddiest Point #3

I looked at Evergreen's website to learn more about OSS but I am a little confused. In class, Dr. He mentioned the criteria of OSS: 1) free distribution; 2) source code available and accessible; 3) people are allowed to modify the code. The FAQ page on Evergreen's site states that the software is OSS but any end-user changes aren't added to the core code and that only specified users are allowed to change the core code. Is this still considered Open Source if there are conditional requirements about when code is adopted into the core or is this "core code" condition the same for all OSS platforms?