thats not true, you can unlock the other path even if you dont do anything with Quinn.Yeah she's the main girl for the others branch.
If you have something with her and go for the others-path, yeah than she is the "maingirl" for that path... well maybe. But its not a given.
For example you can very well play a chick route and go for the "others" path in the end, but a chick-route makes it impossible to have something going with Quinn.