It's not even optional
Look at this:
The key word there is "CURRENTLY".
Possible that they will aren't implementing it as of this moment since it is still new but maybe, just maybe they will add it in the future.
For now, I will not touch my Paypal account in order to buy any Bitcoins in this app. There are many and better options into where to buy Bitcoins and the fact that you can't transfer it as of this moment is already a red flag for me. Maybe if they will allow transferring of Bitcoins then I might decide whether I will try it or not.