clasp
You are right, they gonna create a profile for an account in any case.
A fresh new account is so to say empty and the collection begins from the day it was created.
If you use an older or existing account, the data will be cumulated, but future collection is limited to what slips through Graphenes approach of the sandboxed Google services.
When you create a fresh account this only makes sense if you don't use already known to Google accounts inside Apps and without credit card credentials.
For example if you use social media and use a years old account, Google will connect your fresh account with the old one you abandoned.
Same for email, shopping and what else.
So if you plan to revive old habits on Graphene, you have it easier when you use your old Google account from the start, as you can still use already paid apps and subscriptions.
Graphene will still limit what data Google gets from the OS, but it can only limit it to almost zero when you don't use Google at all.