Android - Does TrueCalller App leak your phonebook?

Yes! it does, when using Truecaller application or services you have already accepted their :

Truecaller - Privacy Policy

So, in this case you agree that :

Truecaller may use your personal information collected by their application to provide, maintain, improve, analyze and personalize their Services for other Users, partners and third party providers.

More specifically, Truecaller may use such information to provide names of contacts from your address book to other Trucaller users.