It will still synch with your Outlook. Once you select the Google synch, you can view all of the other calendars you have (Oulook, Google, Exchange, etc). They only tell you that because it needs to connect with the Google one to allow you to see the rest. In the options, you can filter all of the other calendars out that you do not need by unchecking them.
This app does not replace the built in one, it reads from the same calendar file that is on your phone currently. If you add an appointment in Jorte and then open your built in calendar that came with your phone, you'll see that entry. It's not really a "synch".
EDIT: Once you install Jorte and tell it to synch with Google, while in the app press your Menu button then select Options. In options, click on "Settings". In settings, scroll down to "Default Calendar" and select your PC Synch one. That will ensure that every entry you make goes into that calendar without having to constantly select it.
Hope that makes sense.